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 Mary McDonald B 1857 +/- 4 yrs somewhere in Ireland with the father's forename unknown, I found 115 rcds - too many to summarize here.
Suggest you do some more research to find Sarah and Thomas' exact DOB, and place of birth. English censuses should have this info. Also get the DOB for Mary, her father's forename, and DOB. Then, you can search this database with some chance of success.
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.