Welcome to Ireland Reaching Out!
Unfortunately, the RC records for Kilgeever parish start in 1850. Following your belief that Mary's father was James, I searched on the subscription site Roots Ireland in the Kilgeever records looking for Morrison baptismal records.There was a Thomas Morrison baptized August 29 1853 with parents James Morrison and Bridget Morrison. Also, a Margaret Morrison baptized September 24 1856 with parents James Morrison and Bridget Hunt. Possibly Thomas and Margaret were younger siblings to Mary.
The 1855 Griffiths Valuation head of household listing does show a James Morrison in Killadoon as well as John, Patrick and Thomas Morrison in the same townland. https://www.failteromhat.com/griffiths/mayo/kilgeever.htm