Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
COSKER
As described in historical dictionaries
Patronymica Britannica (1860)
MAC COSKER. Mac Oscar, Ossian's heroic sou. In co. Wexford it is sometimes pronounced Cosgar, and thence anglicized to Cosgrave.