The surname is pronounced Mac Gilla Cuskly and historically referred to in Irish as Mac Giolla Coisgle, there are several current and/or historical English surname variants, including: Cuskelly; Cosgrove; Cosgrave; Cuskery, McCusker; McCasker; McKeusker, McCuskey, McCloskey, Maglosky, McGloskie, McIlCosker, McGilCosker, Mac Cushley and Costello.
Ancient Territory: The Mac Giolla Choiscle were from the old Ui Cremthainn lands around Fintona in the modern parish of Donacavey in Tyrone, which was in the parish of Derrybrusk until the mid 1400s. Many of the Mac Choiscle remained in this area through the plantation to the present. A branch of the Mac Giolla Choiscle were from the 1200s until the plantation in the early 1600s, erenaghs of Derrybrusk who were known as experts in canon law, civil law and harping and clerics in the Diocese of Clogher. The erenagh lands were in and around the townland of Derrybrusk, Fermanagh and also neighbouring Fyagh where the Derrybrusk church ruins are today. Other branches moved through southern Tyrone into Armagh with the expansion of the O’Neills in the mid-1400s, including some Mac Giolla Choiscle retained as the O’Neill’s kern. Another branch moved to Offaly by at least the late 1500s. There are also references to the Mac Giolla Choiscle as husbandmen, weavers and labourers.