I searched the Irish Family History Foundation's database. It contains 12,000,000 surviving rcds for abt 65% of Irish counties. Rcds B4 1840 are sparse.
For Marcella Lynch marriages, I found 3 in Co. Meath: 1842, 1850 and 1856. None of these are in Trim.
For ??? McKeon marriages in Co. Meath, I found 2 which may have been to Marcella. These are a Thomas in 1850 and a John in 1856. None of these are in Trim. Searching at the parish level, I found matching M rcds for Thomas McKeon and Marcella Lynch in Kells parish in 1850. These rcds MAY be for your ancestors. Your chances are good. A view of the detailed rcds should show whether the marriage was to each other, and, if so, their parents' names.
For the children's names, I could only find B rcds for the forename John in the range 1851-1859. Only recently married couples tended to emigrate. Once there were children, the cost became prohibitive. I suspect the children you listed were born in the U.S. You can confirm this through U.S. census rcds.
Suggest you google the Foundation's name to lead you to the database, register, and browse. Pls post your search results to my genforum msg. Good luck.