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 Mar(garet) Scully B 1800 +/- 4 yrs somewhere in Ireland with the father's forename unknown, I found 3 rcds: 1797 Tipperary, 1802 Tipperary, and 1804 Dublin. Father's forenames were Richard, John, and Thomas respectively. You will need an exact DOB, parent(s) name(s), and/or placename to identify which, if any, of these rcds is for your ancestor. Rcd keeping in this early era was very poor.
For Mich(ael) Scully B ditto, I found 1 rcd in 1799 Tipperary.
For John Scully B ditto, I found 3 rcds: 1799 Limerick, 1800 Cork, and 1803 Limerick.
Will leave it to you to find the father's forenames, parishes, and marriage dates in this database.
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.