Peadar Livingstone stated that McOsker – McCusker – MacCosker are principally a branch of the McGuires in Fermanagh, in Northern Ireland.
I believe that we are descended from Oscar Mag Uidhir, the great-grandson of Donn Carragh Mag Uidhir. Oscar was son of Art, son of Flaherty (Flaithbertach) Mag Uidhir, son of Donn Carragh Mag Uidhir.” Uidhir is pronounced “Uwarr”, so Mag Uidhir means “Son of Uidhir”, and sounds like McGuire. Donn Carragh (born in 1264 A.D) is the common ancestor of McGuire, McGwire, Maguire, etc, and is considered the first prince of Fermanagh.
Irish Text of The Annals of Ulster: Matha mac Osgair Megh Uidhir, quieuit in Christo, decimo quarto Kalendas Nouembris, a derbrathair idon Seaan mac Oscair do marbadh isin lo cetna.
English Translation: 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.
Also, I am part of a www.FamilyTreeDNA.com project, and I am genetically matched to many Maguire men from Co. Fermanagh. Our Most Recent Common Ancestor is about 600-700 years ago, and this matches the time frame that Oscar, and his son Matthew, and grandson Maurice lived in Co. Fermanagh.
We have a great deal more information on our website about the beginnings of McCusker / McOsker / MacCosker.