I searched the subscription site Roots Ireland and did not find any McMahon baptismal records for children of Michael McMahon and Judith Griffin in Co. Monaghan. I did find a Maria McMahon baptized September 19 1838 in the Kilcornan RC parish in Co. Limerick with father Michael McMahon and mother Johanna Griffin. (Johanna and Judith were sometimes used interchangeably). I mention this record as a lead in case you might get info that Mary McMahon was not from Co. Monaghan ( despite the grave marker notation).
Have you considerd DNA testing? Possibly you will get some matches that might help with your McMahons and Smith families.