Thomas O'Brien Macmahon was born in Tipperary. A Roman Catholic, he produced two difficult works on human nature: An Essay on the Depravity and Corruption of Human Nature (1774) and Man's Capricious, Petulant and Tyrannical Conduct towards the Irrational Part of Creation (1775). His opinions were promptly ridiculed in the London journals, and he published a sarcastic rejoinder to his critics in 1777. Nothing further is known of him.
From The Continuum Encyclopedia of British Philosophy in Oxford Reference.