I research the Irish Family History Foundation's database. It contains 12,000,000 surviving rcds for abt 65% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 35%.
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 John Carroll B 1830 +/- 4 yrs somewhere in Ireland with the father's forename unknown, I found 285 rcds. 24 have father's forenamed "Thomas." These 24 are the most likely to be for your ancestor. You will need an exact DOB, parent(s) name(s), and/or placename to pick which, if any, rcd is for your ancestor. Find this info in censuses, naturalization papers, church rcds, etc - don't go to the database and guess!
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.