Hi, I mostly have information from 1860s, some before, some after. The LDS (Mormon) Family History Centers have millions of records on Irish as well as worldwide. The centers are free and open to the general public, located in most towns in the USA and elsewhere. Their computerized IGI and addendum diskettes would have the latest; my information is from the microfiched IGI.
To answer your question, I have nothing past 1875 or so, sorry. Do you have any idea of where they might have lived in Co. Sligo or a related surname? Were they Catholic? Since Irish tended to name their children for relatives, that could provide a clue too. Brigida is an earlier form of Bridget, I even found a "Busy Gallagher" who married John Marren, 05 Feb 1853, in Toomour. Keep in mind unusul spellings, such as Gollogher (maybe even Galaher) when you are researching, and many times names were misspelled. Bridget was a common given name.
The Gallaghers lived in the following civil registration areas during the 1840-1870 time frame:
1. Christenings (Catholic) were in Drumcliff and Magherow RC, Calry RC, Aghanagh RC.
2. Other records were recorded in the areas including Castleconor, Toomour, Sligo, Carney, Tobercurry, Cliffony, Kilmacteige, Cloonoghill, Bunnanaddan, Carney, Collooney, Kilmacshalgan, Cloonoghill, Gurteen, Coolaney, (I, personally, don't know if this is a separate district or same as Collooney) from Ballymote, Balingarth, Barratogher, Toomour, Easky, Aclare, Cloonacool. Can someone answer my question?
Some of the surnames connected with the Gallaghers in Co. Sligo during this time period include Boland, O'Hara, Durkan, McEvey, Farres, Lenaghan, Gormon, Devany, Costello, Healy, Noon, Parke, McDonagh, McDonogh, Higgins, Killoron, Fausset, Bree, Gavaghan, Convey, Mulligan, Stenson, McAvan, Hunt, Dodd, Stanton, Mansan, Mullarky, Fancy, Berreen, Marren, Coleman, Lansey, McNulty, Laden, Killoran, Murry, David, Flyn, Doyle, Kirrane, Lang, Mulligan, Quinn, Bradley, Muldoon, Murne, Coegy, Flanagan, Wall, McHugh, McGuinn, Quigley, Grady, Derrig, Hosey, O'Connor, Ohara, Clinton, Walsh, Cauley, Leyden, Judge. Maybe someone can expand this list.
Hope his helps you in some way. Jean