The family fortunes in Ireland peaked in the 1400's when Sir Edward FitzEustace was Lord Deputy of Ireland and his son, Sir Roland, was Lord Treasurer and Lord Deputy being given the title Lord Portlester. Thirty years later, Sir Thomas, nephew of Sir Roland, was made Lord Kilcullen by Henry VIII and then Viscount Baltinglass.His grandson James Eustace, the 3rd viscount, was a devout Catholic and joined the rebellion against Elizabeth defeating the English army at Glenmalure, on the Kildare-Wicklow border, but later lost his lands and titles. James Eustace's properties were forfeited but his life was spared when he fled to safety in Rome.

Many Eustaces died in this tragic campaign but some survived. Today, almost half of the Eustaces in the British Isles are of Irish connection. In the 17th century, Sir Maurice Eustace became Speaker of the Irish House of Commons and Lord Chancellor of Ireland. He was promised the barony of Cry Eustace or Portlester by Charles II but died before it could be bestowed. Eustace Street in Dublin, is on the site of his town house. His cousin, Maurice Eustace of Castlemartin, raised Eustace's regiment for James II, but on the latter's abdication, he was attainted and fled to France. Subsequent generations have had distinguished careers in the armed services, six generals and an admiral. Possibly the most outstanding record is that more than twenty have been High Sheriffs of Kildare and Carlow. That tradition has continued into current generations of the family with Donald D. Eustice of Minnesota having served as the sheriff of Waseca County during the 1970's. Donald was elected president of the Minnesota Sheriff Officer's Association and served in this capacity until his death in the line of duty in 1976. Donald was also president of the Sheriff's Boys Ranch which today is known as the Minnesota Sheriff's Youth Program. A gymnasium located in Austin, Minnesota (USA) is named in his honour. James Eustace/Eustice a native of Ballymahon, County Longford, Ireland emigrated to America with his son John and daughter Bridget in 1850. John married Annie Jewison at Mapleton, Waukesha, County Wisconsin in 1863 and moved first to Michigan and to Minnesota in 1869. They established a family of six sons and one daughter and farmed near Janesville in Waseca County. This family has many descendants in Minnesota and elsewhere including Ronald F. Eustice, editor of the Eustace Families Post. Thomas Eustace was born in County Westmeath, Ireland in 1798, and came to the Georgia/Milton area of Vermont USA in about 1828. He first married Catherine O'Neill in 1831 and after her death in 1832, married Sarah Birney. Thomas Eustace died in Westford, Vermont in 1886.