Historians state that McOsker – MacCusker – MacCosker are principally a branch of the MacGuires in Fermanagh, a county in Northern Ireland, centered at Carrickmacosker.
We believe that we are descended from Oscar Mag Uidhir, the great-grandson of Donn Carragh Mag Uidhir (pronounced Mag Uwarr), the 1st Maguire Prince of Fermanagh.
See www.mcosker.com for more information
(Documented) The Annals of Ulster (compiled in the late 15th century) states the following. "Matthew, son of Oscar Mag Uidhir, rested in Christ on the 14th of the Kalends of November, and his brother John, son of Oscar was killed on the same day. Oscar, son of Art, son of Flaherty (Flaithbertach) Mag Uidhir, son of Donn Carragh Mag Uidhir.”
(Documented) The original Irish text of the Annals of Ulster reads "Matha mac Osgair Megh Uidhir, quieuit in Christo, decimo quarto Kalendas Nouembris, a derbrathair idon Seaan mac Oscair do marbadh isin lo cetna."
(Speculation) Oscar was a potentially important player in the senior Maguire line at the time, as would presumably have been his sons. It is reasonable to assume Matthew and/or John were possibly the stewards of the land now referred to as Carrickmacosker, and around that time changed the family surname to MacOscair (Son of Oscar) (or at least some variant.)