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 Mich(ael) Gannon M 1822 +/- 4 yrs somewhere in Ireland, I found 9 rcds: 1818 Galway, 1818 Dublin, 1820 Meath, 1822 Mayo, 1824 Kildare, 1824 Roscommon, 1825 Galway, and 2-1826 Mayo.
For Marg(aret) Kane M ditto, I found 11 rcds: 1818 Louth, 1818 Longford, 1820 Westmeath, 1820 Louth, 1821 Dublin, 1822 Roscommon, 1822 Clare, 1824 Cavan, 2-1826 Tyrone, and 1826 Westmeath.
As you can see, there are no yr/cnty combinations in common. This infers: 1) the search range was wrong; 2) the rcds didn't survive; or 3) the rcds haven't been transcribed.
For Mich(ael) Gannon B 1805 +/- 4 yrs somewhere in Ireland with a father forenamed Mich(ael), I found 1 rcd in 1801 Meath. Rcds from this era are sparse. Most events either weren't rcd'd, or the rcds didn't survive. You chances are slim.
For Michael Gannon M 1800 +/- 1 yr somewhere in Ireland, I found 2 rcds: 1799 Dublin, and 1800 Galway.
For Mary Costello M ditto, I found 2 rcds, but none in the same counties as for Michael.
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.