His close friend was Peadar Ó Doirnín, a fellow-poet with whom he founded a hedge school teaching Irish bardic poetry.
In the summer of 1744, Mac Murchaidh and O'Doirnin organised a monster meeting on Slieve Gullion to motivate the local people for the imminent arrival of Prince Charles Edward Stuart.
However, their affair was tempestuous; after a fierce argument and break-up, Molly allegedly swore revenge on Mac Murphy.
To this end, she plied Ó Doirnín with drink one evening, and inveigled him to compose a satirical poem about Johnston called The Heretic Headhunter.
However, another version lays the blame on a lieutenant of Mac Murchaidh, Art Fearon, who wished to ingratiate himself with Molly.
His body was left hanging for three days before it was taken down, waked for two nights at his mother's barn in Carnally, before been buried in Creggan churchyard.
For violating the traditional Irish code of silence regarding both the police and the courts, Molly MacDecker was ostracised by her community, and became mentally ill. She eventually drowned herself at Narrow Water.