I searched the Irish Family History Foundation's database. It contains 11,500,000 survivng rcds for abt 60% of Irish counties. Rcds B4 1840 are sparse. However, some Co. Louth rcds date to 1744.
For John Kerns (+variants) B abt 1820, I found 3 B rcds: 1814, 1820, and 1825. Only the 1820 rcd lists Drogheda as the parish. The rcd for 1820 spells the forename as "Jno." It is reasonable to infer that this was intended to be John. The error was either made by the original recorder, or the IFHF transcriber. A view of the detailed rcd SHOULD list the parents' names. The 1820 rcd is your best shot.
If the parents' names are listed, you can search for the marriage date. If one is found, the M rcd MAY list their parents' names. You may also be able to infer their DOB.
Google the Foundation's name to lead you to its database.
Post a msg to Genforum to tell me the results of your search.