I research the Irish Family History Foundation's database. It contains 12,800,000 surviving rcds for abt 70% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800. Cos. Donegal and Derry were recently added to the database.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 30%.
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 Henry Breen B ????, Co. Cavan, with a father forenamed John, I found 0 rcds.
For Mary Nixon B ????, Co. Cavan, with a father forenamed John, I found 0 rcds within a relevant range.
For John Breen M 1845 +/- 4 yrs, Co. Cavan, I found
For John Nixon M 1845 +/- 4 yrs, Co. Cavan, I found 1 rcd in 1844, Castletara CI parish.
For Mary Cordock M 1844 ditto, I found 0 rcds matching that for John. This infers John's rcd is a false positive.
For John Breen M 1845 ditto, I found 1 rcd in 1845, Killan RC parish.
For Eliza(beth) Kithcard/Kithcart M ???? somewhere in Ireland, I found 0 rcds for any date.
These negative results suggest that the B/M rcds for these individuals either didn't survive, or haven't been transcribed yet. Suggest you wait until the database is more complete.
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.