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 John McGowan B 1884 (exact) in Co. Sligo with the father's forename unknown, I found 3 rcds. Parishes and father's names are as follows: Ahamlish [John], Drumcliff [Patrick], and Templeboy [John]).
LEITRIM Mary K. 1915 Anna J. Bridget 1910, 1911 James 1918 Helen
For the marriage and children, I can't find a common parish and set of years with a father's name of John.
B/M/D registration was civil after 1863 and comprehensive. Civil rcds are kept by the General Register Office. Contact it at http://www.groireland.iehttp://www.groireland.ie for cert. copies. Fees apply.
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.