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 county, 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 Arthur Fegan B 1872 +/- 1 yr, Co. Down, with a father forenamed Bernard, I found 1 rcd in 1871.
For Pat(rick) Fegan B 1869 ditto, I found 3 rcds: 2-1869 and 1871.
For James Fegan B ditto, I found 3 rcds: 1868, 1872 and 1873.
None of these quite match your info. Therefore, there is a good chance these are all false positives. Since B/M/D registration was civil after 1863, civil rcds may still exist. Contact the Public Record Office for Northern Ireland (Google PRONI). Fees apply.
For Bernard Fagan M 1865 +/- 4 yrs in Co. Down, I found 6 rcds: 3-1864 and 3-1866.
For Sarah McLinden M ditto, I found 11 rcds with 5 in 1864 and 1 in 1866.
Will leave it to you to see if any of these rcds also match at the parish level. If so, they are highly likely to be for these ancestors. A view of the M rcds SHOULD show they married each other and list their parents' names and other info.
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.