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 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 Bri(dget) McHugh B 1869 +/- 4 yrs in Co. Roscommon with a father forenamed Thomas, I found 1 rcd for "Brigid" in 1866, Boyle RC parish. A view of the B rcd will likely show BOTH parent's names, usually including the mother's maiden name. If so, this will allow you to search for the Marriage rcds with a good chance of identifying them.
For Thomas McHugh B 1826 +/- 4 yrs in Bolyle parish, Co. Roscommon with the father's forename unknown, I found 4 B rcds: 1825, 2-1827, and 1829. Boyle's B rcds start in 1793. Searching throughout Co. Roscommon, I found only 1 additional rcd in 1826. Suggest you work (successfully) on the marriage rcds before attempting to identify the correct B rcd for Thomas.
Possible siblings born in the same parish include Elizabeth 1870, Michael 1863, and Patrick 1863.
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.