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 Thomas Scully B 1825 +/- 4 yrs somewhere in Ireland with the father's forename unknown, I found 12 rcds: 1821 Galway, 2-1822 Kilkenny, 1825 Dublin, 1825 Cork, 1826 Tipperary, 1827 Kildare, 2-1828 Cork, 1829 Roscommon, 1829 Longford, and 1829 Tipperary. You will need an exact DOB, parent(s) name(s), and/or placename to determine which, if any, of these rcds are for your ancestor. Suggest you look at censuses, naturalization papers, church marriage rcds, and civil death rcds to find this info.
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.