Mac Fhirbhisigh was the surname of a family of Irish hereditary historians based for much of their known history at Leckan, Tireagh, Co. Sligo. They claimed descent from Dathí (d.418?/428?), said to be one of the last pagan Kings of Connacht, and were thus one of the many families who sprang from the Uí Fiachrach dynasty.
Based initially at Cong, the family may have been erenaghs prior to becoming historians and genealogists to the Ui Fiachrach Muaidhe (branch of the Uí Fiachrach based in north Connacht) and several other patrons. While this was their main function, they also appear to have served as poets and brehons to their patrons, possibly even musicians.
Among the earliest known members of the family were Amhlaoibh Mór mac Fir Bhisigh (died 1138), Domhnall na Sgoile Mac Fir Bhisigh (fl.c. 1250) and Gilla Isa Mac Fir Bisigh, who died in 1301.
In the late medieval era the most well known was Giolla Íosa Mór Mac Fhirbhisigh fl. 1390-1418. Fear Dorcha MacFhirbhisigh (fl.c. 1600?) was noted as well-travelled in Scotland, showing how far afield members of the clan would venture in practiseing their art.
Dubhaltach MacFhirbhisigh (fl.c.1640-January 1671) compiled Leabhar na nGenealach, a massive compilation of Irish genealogies, a collection of much lore that would have been lost without trace in the turbulent Irish 17th century.
The name is now rendered as Forbes.