Family Search births and baptisms 1620/1881 is showing births for children of a similarly named couple,
Ellen O'Mullan 11 sep 1876 dtr of Jeremiah O'Mullan and Elizabeth O'Kane,Limavady,Derry.
Fathers birthplace Ashlamaduff.
it also shows a birth for George on the 16 Feb 1865,Garvagh,Londonderry.
Perhaps if you got Georges birth cert it would lead you to church files microfilm for the baptism.
Being a southern boy myself i dont know much about the lay of the Land in Londonderry.