Gleeson, a farmer, actually was the one to report McCarthy's death to police in the first place. He found her body in a field. McCarthy, an unmarried mother of seven, had been shot in the face twice with a shotgun.
The Griffith College-based Irish Innocence Project—part of the larger, global organization of the same name—and the Justice for Harry Gleeson Group submitted research to Ireland's Department of Justice last year which revealed that the prosecution in Gleeson's trial had withheld information, beat one witness and encouraged several more to falsify statements. According to the Times, a pathologist also verified Gleeson had an alibi using forensic evidence.
The Tipperary Star reports that McCarthy was known as "Foxy Moll" for her "flaming red hair." Apparently, local gossip held that Gleeson was the father of one of McCarthy's children. (Gleeson, a bachelor, denied this.) From the Star:
A social outcast, Mary kept her head high, even boasting to local women about all the men whom she "entertained" in her two-room cottage. Reviled and feared in equal measure, Mary was reputed to keep a list of local men who visited and lusted after her.
This, along with her refusal to adhere by the puritan standards of the time - the parish priest described her as the "Devil's Disciple" - made her a target among those who feared their own names might leak out.
So Harry Gleeson gets his pardon 74 years too late, and we still don't know who killed Mary McCarthy.