I have wondered if Samuel was economical with the truth in other areas, too.... with a name like Gallahar, perhaps he was born in Ireland and preferred to be thought of as Scottish (which might explain the name change). The large-scale immigration from Ireland in the 1840s didn't do a great deal for English/Irish relations.
I haven't come across any avoidance of Irish roots in the past, but other Irish people I've researched have been in Liverpool, where there were always a large number of them in a small area, perhaps resulting in less persecution (certainly the black population of Liverpool were readily accepted as part of the community). Could it be that Samuel said he was Scottish and changed his name once he came into Cumberland because he wanted to shake his Irish origin and avoid discrimination?
Unfortunately, I will have to re-register with Scotland's People, as I forget my login details... but I can't register at the moment because my mail service is down until this afternoon for "maintenance"
The 1792 entry looks quite good for Mary.... it's close to her age in 1861 (though not for 1871)... but John and Jean were her firstborns' names... if Samuel was from Ireland, or some distance from his family... it's possible they may have named their first 2 after her parents if they lived closer.