Sean Matgamna, also known as John O'Mahony (the English language equivalent of Sean Matgamna) is a Trotskyist theorist. He was born in 1942 in County Clare in the Republic of Ireland.

He joined the Communist Party of Great Britain when he moved to Manchester to work as a docker , while still in his teens. He soon grew disillusioned with what he saw as the Communist Party's reformist stance, and joined the more openly revolutionary Trotskyist Socialist Labour League, led by Gerry Healy.

In the group, Matgamna soon became the youth organiser, but came to disagree with Healy over a number of issues, including Healy's anti-gay rights stance. As a result, Matgamna was expelled in 1964 and with a small number of supporters joined the other Trotskyist group operating in Manchester, the Revolutionary Socialist League. He soon grew dissatisfied with the group, feeling that they had moved away from orthodox Trotskyism and the International Secretariat of the Fourth International. He wrote a pamphlet, What We Are and What We Must Become, outlining his views. When the RSL declined to publish it, he distributed it himself and was as a result expelled from the organisation in 1966.

Matgamna, working with two supporters, formed the Workers Fight group to act upon his views, central to which was a call for Trotskyist unity in Britain. A handful of other RSL members joined the group before, in 1968, the International Socialists also put out a call for unity. Responding to it, Workers Fight joined the IS as the Trotskyist Tendency.

When the IS leadership forced Matgamna's tendency to leave the IS, it took with it a much increased membership. Martin Thomas soon joined, working with Matgamna to take prominent roles in the group. Matgamna became a full time theorist within the group, moving to London.

In the 1980s, Matgamna, along with many other members of the group, by then known as the Socialist Organiser Alliance, came to reconsider some of his views. Rereading works by Hal Draper and Max Shachtman led him to conclude that Third Camp socialism offered an eloquent expression of many of the conclusions he had come to.

Matgamna is still a prominent member of the Trotskyist group he founded, now known as the Alliance for Workers Liberty.