My maternal grandfather was named James Gallagher, and he grew up in County Mayo (Killedan parish). There are actually two main Gallagher "clans". The largest originated in County Donegal, and the other in Counties Galway and Mayo (the counties were a somewhat late creation, and the clan names date from centuries before the counties were created). They are said to be unrelated families, although I have a number of DNA matches in Donegal as well. I also have some DNA matches in Nova Scotia, but I don't know the precise connection there, so I'm not sure whether it's a Gallagher connection (my maternal grandmother and 3 of my 4 paternal great-grandparents were also born in Ireland).
As you noted, in Irish, Gallagher is Ó Gallchobhair (reformed spelling Ó Gallchúir), and it means “descendant of the foreign help”, most likely referring to real or legendary mercenary warrior ancestors who were brought in from Scotland or elsewhere in Ireland to fight in local wars (known as a gallóglach, or “gallowglass”). It may also refer to Norse ancestors, since the term “gall” was used for them as well. Although Gallagher and Gallaher are by far the most common anglicized forms of the name, especially in Mayo and Donegal, there are more than 20 other historical anglicized forms scattered around.
You're searching for people in the 1800's, so DNA testing might well help you to locate a connection with someone who can tell you more about your specific family. I used it successfully to locate several branches of my family about whom I had only a bit of info.
