I searched the Irish Family History Foundation's database.
For Owen McMahon, B 1799, I found 0 B rcds.
For Owen McMahon, M 1835 +/- 4 yrs, I found 1 M rcd in 1839 in Ardee parish. A view of the detailed rcd MAY show his wife's maiden name.
For Mary McMahon, I found 1 B rcd, 1841, Ardee parish. For Catherine McMahon, B 1842 +/- 4 yrs, I found 1 B rcd in 1840, Dundalk parish. For John McMahon, B 1845 +/- 4 yrs, I found 0 B rcds. These MAY, or MAY NOT, be your ancestors. You will have to view a detailed rcd to see if each lists the correct parents.
Google the Foundation's name to lead you to the database.