Thanks Aghadowey.
I cant find any marriage between a Barney/Bernard Gallagher and a Grace. She would have been about 17 when the first child was born. The best thing would be to get one of the children's birth certificates as that will give her maiden name.
I'm having problems though finding children registered where it says on the 1891 census - I wonder, if they weren't married, if they were registered in her name?