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 ONLY available thru the Tipperary Family History Research center at a substantial fee (regardless of outcome).
For Tho(mas) Breen B 1850 +/- 2 yrs, somewhere in Ireland, with the father's forename unknown, I found 8 rcds: 1849 KIL, 2-1850 KIL, 1850 CRK, 1850 TIP, 1852 FER, 1852 KIL, and 1852 LOU. You will need an exact DOB, parent(s) name(s), and/or placename(s) to pick which, if any, is the correct one. The forename of his oldest son can sometimes be used to infer the name of Thomas' father.
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.