I have a host of Gallachers that, aside from one, I have never been able to pin down. My ggg grandmother, Isabella (Gallacher) Brown (b. IRL, d.1857, Bonhill). On the 1851 census boarding with the widow Brown was a Daniel Gallacher, age 28, b. IRL, and Ann Gallacher, age 25, b. IRL.
Isabella's daughter, Mary Brown married William Waugh in 1860 in Alexandria, with one witness a Finegal (or Ferigal) Gallacher. Boarding with William and Mary on the 1871 census was a Nancy Gallacher, age 45, b. IRL. On the 1881 census was boarder James Gallacher, age 25, b. Kilmarnock.
In 1887, William and Mary's 2nd daughter Isabella Waugh married William Gallacher, b. Houston and Killellan, son of perhaps the above Daniel.
Probably all the above are related, but I've never managed to 'link' with any surety.