I searched the Irish Family History Foundation's database. It contains 11,500,000 surviving rcds for abt 60% of Irish counties. Rcds B4 1840 are sparse.
For Thomas, the database spells the surname "Mahan." For Mary, the maiden surname is spelled "Donegan."
Separate marriage rcds for Thomas & Mary exist in the same county, year, and parish. This is a VERY close match. It is doubtful if it is due to random chance. I suggest you view the detailed rcds and print them - preferably on high rag content paper.
For each of the listed children and associated DOB, a B rcd exists in Mohill parish. For Mary Mahan B 1854 +/- 4 yrs, I found 4 B rcds: 1853, 1854, 1856, & 1858. You will have to view detailed rcds until you find the one which has the right parents' names. Start with the yrs which don't conflict with the gestation periods for the known DOB. Suggest you view and print the detailed rcds for posterity.
For the names Katherine or Kathleen Mahan, I found 0 B rcds in this county and parish for ALL years. However, there is 1 B rcd for the spelling Catherine "Mahon" in 1864. This fits nicely into the birth chronology. The difference in spelling is likely due to an initial recording error, or an IFHF transcription error. In any event, spelling was flexible in the 19th century. It didn't matter because most poor Irish could neither read nor write.
I noticed the 1st Elizabeth must have died. They named a 2nd in her memory. Her death was not rcd'd. Death rcds were the least kept.
Thank you for posting this msg. You have perhaps the best organized and complete data I have ever seen. Contact me when you start searching for Thomas and Mary's birth dates.
Google the Foundation's name to lead you to the database. You can duplicate everything I have done for free. But, if you want to view a detailed rcd, the fee is 5 euros each. Serious geneology doesn't come cheap.
Post a msg to Genforum to let me know your search results.