I don't know much about Leitrim Mechams. I do know there is a variety of surname variants. Some time ago, someone sent me some names and places that suggest to me a possible association with the family of your Ann/a Mecham.
I wouldn't expect to find your Mohill ancestors had spelt their name Mecham, more likely Meacom, or Meakim, or one of the other versions. But, I'm no expert.
Some of the names I have are: Jane Meakin (d. 1878) and her sister Catherine (d. 1859), married (1828) to Charles and William Jones of Taish.Daughters of Morgan Meakon from Lurga (near Mohill). Suggest email me off-forum if you want to further discuss.