Mac Giolla Iasachta
Mac GIOLLA IASACHTA—IV—M'Gillesachta, M'Gillisachta, M'Gillysaghtie, MacLysaght, Lysaght, Lysat; 'son of Giolla iasachta (strange youth); a well-known Clare surname. The family is said to be a branch of the O'Briens, descended from Domhnall Mór Ó Briain, King of Munster (1163-1194).
Alphabetical Index to Irish Surnames