I research the Irish Family History Foundation's database. It contains 12,800,000 surviving rcds for abt 70% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800. Cos. Donegal and Derry were recently added to the database.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 30%.
Note 2 - Only 1/2 of Co. Cork's rcds are in this database.
Note 3 - The RC church kept control over most parish rcds from Tipperary (north). These are available thru the Tipperary Family History Research center at a substantial fee (regardless of outcome), and the National Library of Ireland on microfilm.
For Michael Carroll M 1870 +/- 1 yr, somewhere in Ireland, I found 31 rcds: 2-1870 ROS*, 1870 LOU, 2-1870 LIM*, 3-1870 TIP, 2-1871 GAL, 4-1871 MAY*, 4-1871 LIM, 2-1871 TIP, 1871 WSM, 1871 DON*, 4-1869 LIM*, 2-1869 TIP, 2-1869 DUB, and 1869 SLI.
For Br(idget) Brown M ditto, I found 20 rcds: 1870 GAL, 1870 DON, 2-1870 MAY, 2-1870 LIM*, 2-1870 ROS*, 2-1871 LEI, 1871 DON*, 1871 CRK, 2-1871 MAY*, 1869 LIM*, 1869 CLR, 1869 MAY, and 3-1869 LIM. Only the asterisked yr/cnty combinations have POTENTIALLY matching rcds. These must be further compared to see if there are matches at the parish level. If so, these are highly likely to be for these ancestors.
Google the Foundation's name to lead you to this database. Register, opt to be notified as rcds are added and browse. Pls post your search results to my genforum msg.