I searched the Irish Family History Foundation's database. It contains 11,500,000 surviving rcds for abt 60% of Irish counties. Rcds B4 1840 are sparse.
No dates! For Thomas McDonagh in all Ireland, I found 595 B rcds and 266 M rcds. For Co. Roscommon, the numbers are 61 and 26 respectively. One of these MAY be for your ancestor, but the others are false positives.
For Ann Walsh, there are 2,252 B and 1,024 M rcds in all Ireland. For Co. Roscommon, the numbers are 68 and 33. Ditto the above.
Searching for marriages is usually more fruitful than births. You need to find where M rcds exist for each name in the same yr, same county, and same parish. For this, you need an estimated marriage date. Generally, couples emigrated shortly after marriage and B4 they had any/many children. If you know their emigration date, assume they married within 2 yrs of that date. Females married between 19 and 23 yrs old, males 21 to 25.
Google the Foundation's name to lead you to this database. Pls post your search results to my genforum msg.