McCusker is a surname of Gaelic origin used predominantly in Ulster (Fermanagh, Tyrone, Derry, and Armagh). Often interchangeable with Cosgrove, which was more common in parts of Armagh and in Cavan and Longford.
McCusker, Cosgrove and other similar surnames are anglicisations of the Gaelic surnames of a number of unrelated Gaelic families of Ulster, including the:
- Mac Giolla Coisgle or Mac Giolla Coisgli or more modernly Mac Giolla Choiscle, an old Barony of Tirkennedy, Fermanagh family. Brehons (both Irish and canon law), Harpists, Erenaghs of Derrybrusk in Fermanagh, high ranking churchmen in the Dioceses of Clogher and Armagh including abbots, culdees, canon choral, canons, vicars, priests, lawyers and husbandmen (anglicised to McCusker, McCosker, Cosgrove, McCosgrove, Cuskery, McCuskey, Cuskelly, McCloskey and others). Spread through South Ulster (Fermanagh, Tyrone and Armagh) and Westmeath/Offaly branch.
- Mac Coscraigh, Erenaghs of part of the lands of the Abbey of Clones, Clones Civil Parish, Monaghan
- Mac Oscar of Magheracross and Derryvullan, Fermanagh and later Dromore (Tyrone), a branch of the Maguires, descending from Oscar Maguire
- Mac Coscraigh of Cenel Moen, Donegal/Tyrone
- McCoskerans or McCosrichans of Down
- McCoskerans of Ballinderry, DerryNotable people with the surname include: