I searched 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 parish, 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 Oliver Williamson B 1799 +/- 4 yrs in Co. Cavan with the father's forename unknown, I found 0 B rcds. There are NO Oliver Williamsons B rcds in Cavan or anywhere in Ireland, ever! The same is true of marriages.
For Thomas Williamson B 1779 ditto, I found 0 B rcds.
At this early date, there are virtually 0 B/M/D rcds in Cavan.
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.