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).
Note 4 - The National Library of Ireland (NLI) has microfilms of parish church rcds.
For Mal(achy) Fenneran B ????, Co. Roscommon, with the father's forename unknown, I found 0 rcds. This is the broadest possible search. Note - RC parish registers after 1900 aren't in the database. Also, many civil rcds, which began in 1864, were destroyed in the Four Courts fire in 1922. Since Malachy is the youngest, rcds of his older siblings may exist. Suggest you repost with their names and DOB, or visit the database and search for yourself.
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.