The first recorded spelling of the family name is shown to be that of Fitzgerald (Gaelic form Mac Gerailt), which was dated c1400, in 'The Annals of Ireland'.

In Leinster the head of the main branch became the Earl of Kildare and then the Duke of Leinster and was the foremost peer in Ireland. There were twenty Earls of Kildare from 1316-1766 when they became Dukes of Leinster. The Kildare family was very powerful and had castles in Maynooth and Kilkea. One of the most illustrious of these was Garret Mor Fitzgerald, eighth Earl of Kildare (deceased 1513), of whom Henry V11 is reputed to have said 'All Ireland cannot govern this Earl; then let this Earl govern all Ireland'.

The Munster branch who served as Earls of Desmond, were located at Dingle and were known as the Knights of Kerry. Between the years 1329 and 1601, sixteen Fitzgeralds held the title 'Earl of Desmond'. They were estimated to have owned one million acres of land and they had castles in the baronies of Coshmore and Coshbride in Waterford.

The Fitzgeralds of Kilkenny were transplanted to Mayo in the 17th century and settled in the parish known as 'Clanmorris' (now Claremorris), where their chief was called MacMorris.