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 - without a known county, there is AT LEAST a 35% chance your ancestors rcds aren't in the database.
For Catherine McCarthy B 1800 in Roscommon, the earliest B rcds for this name is in 1830.
For Bernard McDonough B 1820 +/- 1 yr in Roscommonwith a father named Pat(rick), I found 0 B rcds.
For Margaret " B 1821 ditto, I found 0 B rcds.
For John " B 1823 ditto, I found 1 rcd: 1824.
For Catherine " B 1825 ditto, I found 2 B rcds: 1825, 1826.
For Thomas " B 1825 ditto, I found 0 B rcds.
Note - a study of the Roscommon "sources" on this website shows that less than 1/2 of Roscommon parishes were keeping rcds during this range of yrs. Since I get mixed results for the children's births, it is likely they were B in a parish which wasn't keeping rcds.
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.