The next known owner of Castlemartin was the famous Sir Edward FitzEustace, who was the son, or possibly the grandson, of Roland. If a grandson, his father was probably John FitzEustace, one of the twelve delegates chosen under Letters Patent of 1404 to control the financing and organization of the defense of the County. Thomas of Ballycotelan and John of Blackhall were also among the delegatesSir Edward FitzEustace of Castlemartin was High Sheriff of County Kildare in 1421, 1423, 1425, 1426 and 1430, and was appointed a Privy Councilor in 1431 when he went to report to the young King Henry VI. When the Duke of York was appointed Viceroy of Ireland in 1449, Sir Edward was well known as a knight of vigor and activity, and was made Lord Deputy during the absence of the Duke in 1452, and again in 1454 when he died in office. In 1446 Castlemartin was unsuccessfully attacked by Irish tribesmen under O’Connor Faly. From 1445 Sir Edward held from the Crown, on death of Elizabeth Calfe, Baroness of Narragh, parts of the Barony of Clane and Ladytown, Old Connell, but the grant was rescinded in favour of John Bellow." There would seem to be external evidence that the effigy in chain mail now lying in Ballymore Eustace church is that of Sir Edward, but some authorities rather surprisingly) date it somewhat later. (Further details of this effigy are given under "Kilcullen"). It was originally at Old Kilcullen, the church of Castlemartin before the chapel was built, probably by Sir Edward’s son. By his wife, Alicia, Sir Edward had at least four sons. There are now no means of telling which was the eldest, but in the opinions of the present writer the most probable order was Sir Thomas of Castlemartin; Sir Roland, later Lord Portlester;  Richard, father of the 1st Viscount Baltinglass and Oliver, High Sheriff of Kildare 1445.

Sir Thomas of Castlemartin had been knighted by 1438, but we know very little about him, and in fact for the next century the Eustaces of Castlemartin were overshadowed by those of Harristown, descended from Sir Thomas’s brother Roland Lord Portlester. Thomas was buried at Castlemartin in the centre of the little chapel on the high wooded banks of the Liffey. His effigy in armour lay on the top of an altar tomb which was surrounded by panels representing various biblical and ecclesiastical figures, and with one panel bearing his coat of arms—Or a saltire gules. The effigy now lies broken and headless on the overgrown floor, and the remains of the carved panels are piled at one end of the chapel. The vault below the tomb when opened was found to contain eleven skeletons, one of the skulls having a clean cleft made by a sword cut. They were reverently re-interred in the vault. We know that many Eustaces of Castlemartin, Harristown and Clongowes Wood were buried here, but no other Eustace tombstones now remain.

Sir Thomas had a son Edward, the father of John of Castlemartin, who in 1505 was granted the Castle of Inchcoventry; with certain lands there and at Ballyculane, to hold for the use of the son of the 8th Earl of Kildare, who had married the daughter of John’s great-uncle Lord Portlester. He had a son Maurice of Castlemartin, who at the time of the Reformation became a Protestant, as were all his descendants except a very few who will be noted as devout and active Catholics when they occur later in this history. We shall return to the House of Castlemartin, but must first consider the far more famous (at this time) House of Harristown, which remained ardently Catholic, though Harristown, itself passed to the Protestant Castlemartin branch after the Baltinglass rebellion of 1580. In 1547 Maurice had been granted Brannockstow'n, Rochestown and Boleybeg (just south of Harristown), forfeited by Christopher Eustace of Ballycotelan after the "Silken Thomas" rebellion, to be mentioned shortly (see also under Coghlanstown and Brannockstown).

Sir Roland FitzEustace of Harristown, later Roland Eustace, Baron Portlester

Roland, son of Sir Edward, the Lord Deputy, was born about 1430, and was destined soon to become one of the principal men in Ireland during the Wars of the Roses. He was trained as a barrister, and by 1454 had been appointed Chief Clerk to the King's Bench and Keeper of the Rolls. Later that year lie was chosen by the Viceroy, the Duke of York, to be Lord Treasurer, a post he held for thirty-eight years. He was knighted in 1459, and in 1462 was created by Edward IV, Baron Portlester. (Portlester was one of the first Irish peers to be so created by Letters Patent, the only peerages so granted before 1500 being those of the Earls of Ulster, Carrick, Kildare, Louth, Ormonde, Desmond and Waterford; Viscount Gormanston 1478; and Barons Trimleston 1462, Portlester 1462 and Ratowth 1468.)  In 1463, he was appointed Lord Deputy to the absent Viceroy, the Duke of Clarence. He was Lord Chancellor from 1472 to 1480 and again from 1486 to 1492. He was Captain of the Brotherhood or Guild of St. George, a body constituted by Act of Parliament in 1472 for the better defense of the Pale. It was headed by the 7th Earl of Kildare, under whom were the elected Captain and eleven other peers and knights, with 120 mounted archers, 40 horsemen and 40 pages. They had power to make laws and to arrest rebels, and were not dissolved until 1494. In 1473, Portlester became a Member of the Fraternity of Arms.

In 1467, Portlester had narrowly escaped execution. The Queen, who had been offended by the Earls of Desmond and Kildare, contrived that the Earl of Worcester should be sent to Ireland as Lord Deputy. Desmond was at once quite unjustly accused and beheaded; Kildare was attainted; and Portlester charged with treason. He offered wager by battle, but his accuser, Sir John Gilbert, fled, with the result that Portlester was completely exonerated by Parliament and Sir John attainted. Worcester himself was beheaded on Tower Hill in 1470.

The great 8th Earl of Kildare succeeded his father in 1477 and was appointed Lord Deputy. Portlester, although many years his senior, became his firm friend and later his father-in-law. The Earl so on held the famous Parliament of Naas which refused to recognize the King's representative, Lord Henry Grey. He and Portlester found themselves in serious trouble, but were eventually forgiven, although Portlester was replaced as Lord Chancellor by the Bishop of Meath. He refused however to hand over the Great Seal to his successor, and another had to be made before the affairs of State could be carried on.

Kildare and Portlester were ardent Yorkists, and after the Lancastrian victory at Bosworth in 1485 they regarded the new King Henry VII as merely an illegitimate Welsh adventurer. When, therefore, there arrived in Ireland the Yorkist claimant, Lambert Simnel, who after a thorough examination appeared to be the undoubted son of the Duke of Clarence, he was crowned at Dublin in 1487. Kildare and Portlester went to England to support the Yorkist claim, but it ended disastrously at Stoke on-Trent. Once again they received a royal pardon, Portlester being confirmed as Lord Treasurer and once more as Lord Chancellor- by the Tudor King. He seems to have previously assigned, in 1482, the office of Chief Baron of the Exchequer to his son, Oliver. In 1492, the quarrels among the Anglo-Irish at last enabled Henry to displace Kildare and his -Council, and Portlester soon found himself threatened with a hostile enquiry into the Treasury accounts. He had however run his course, and died in 1496.

Portlester was a generous benefactor of the Church. In 1455, he, added the Portlester Chapel at the east end of St. Audoen's Church, then the wealthiest parish in Dublin, and in 1486, he founded the Franciscan New Abbey of Grey Friars at Kilcullen. He was a benefactor of St. Malcolyn's, Hollywood (three miles south of Ballymore Eustace), and a co-founder of the Guild and Chantries of St. Columb, Skreen, and the Chantries at Piercetown, Laundey and Greenoge. He refounded the Guild of English Merchants Trading in Ireland.

He married three times. First, Elizabeth, daughter of John Brune; second, in about 1463, Joanna (or Joan),' widow of Christopher Plunkett, 1st Lord Killeen, and daughter of Bellew of Bellewstown ( Her young grandson, Broughton Plunkett, accompanied Kildare to England in 1487, and was killed at the Battle of Stoke); and third, in about 1467, Margaret, widow of John, the son of Sir John Dowdall of Newtown, and also the widow of Thomas Barnewall, daughter and co-heiress of Jenico D'Artois. She predeceased him, and was buried at St. Audoen's, Dublin.

His two sons, Oliver and Sir Richard, perhaps illegitimate, predeceased him Richard by only a year or so. He had four daughters, probably all by his third wife: Alison, (She was the mother of the 9th Earl, and the second of her six daughters was the famous Great Countess of Ormonde and Ossory) who married 8th Earl of Kildare; Joan, (The Complete Peerage (presumably with good reason) gives two Joans, and assigns the first and also Alison to the second wife. Neither of these assumptions seems to me to be at all likely) who married (1) her cousin Richard Plunkett, 2nd Baron of Dunsany, and (2) her second-cousin Sir Maurice Eustace of Ballycotelan ; Maud, who married (1) Thomas Marward, Baron of Skreen, Co. Meath, (d. 1504) and (2) Sir John Plunkett of Bewley, Co. Louth; and Janet (d. 1536), who married Sir Walter Delahyde of Moyglarie, Co. Meath. She, with her two sons, James and John, and Thomas Eustace her nephew, ( A younger son of her sister Joan, whose eldest son, Christopher, was executed in 1535) was accused of having incited the 10th Earl of Kildare to rebel in 1534, and was detained in Dublin Castle as " Dame Jenet Eustace, the traitor's aunt and foster mother."

Portlester presumably came into possession of Harristown as his share of his father's Castlemartin property. He built (or added to) the castle, no doubt as part of the activities of the Guild of St. George. There was at one time a second, perhaps the original, castle on the estate, but its site is unknown. Little now remains of Harristown castle, but the earthworks (just west of the railway station) show that it must have been an imposing structure. The estate probably extended slightly to the east of the present townland boundary, so as to include the little chapel of St. James in Coghlanstown, then the property of the Eustaces of Ballycotelan where stands the remains of Portlester's memorial cross. An old map of about 1645 shows that the townland boundary was in its present position by that date. Among the other castles built by Portlester was that of Balablaght, at the request of the Abbot of Baltinglass.

Portlester was buried at New Abbey, where his daughter, Alison, Countess of Kildare, had been buried the year before. She is said to have died of grief when the 8th Earl was arrested in 1495 and imprisoned in the Tower. She was not to know that he would be freed the following year and appointed by the King " to rule over all Ireland," as the result of the famous trial.

Portlester's tomb in the chapel of New Abbey must have been strikingly similar to that of his brother at Castlemartin, but it bore two effigies-himself in armour, and Margaret in a long belted and pleated costume, wearing a " horned "' head dress. One panel bore his arms-the Eustace saltire (differenced by an annulet, denoting a fifth son, which may have been intended for a crescent, denoting a second son. The carving of this panel is inaccurate in that the arms of D'Artois should be " Barry wavy of six." Note that on the death of Margaret (an heraldic heiress) he inherited and quartered her arms, but this quartering died with him), quartering D'Artois (carved as " Barry of six "). In St. Audoen's church in Dublin, and now placed for protection under the ruined tower, there is a tomb or cenotaph bearing almost identical effigies and inscription, but now without the side panels. This was probably originally placed over Margaret's grave. These three tombs and their effigies will be further considered under Kilcullen." as will the Eustace effigy at Ballymore Eustace. In the graveyard (now moved to inside the church ruins) of St. James's chapel, Coghlanstown, there are the base and shaft of a memorial cross bearing his saltire with baron's coronet and what are probably the arms of his first two wives. The inscription " Eustace Lord Portlester 1462" appears from the style of the lettering to have been added in the seventeenth century.                

On his death in 1496, Thomas, his nephew, son of his brother Richard, succeeded to his estates but the title of Lord Portlester became extinct.