Green’s Dictionary of Slang

(Irish) a self-interested individual.

1987		L. Swords Topical Homilies 171: He was what is known in some places as a ‘mé-féiner’.
1999		q.in J. McCarthy Mary MacAleese 200: ‘Let me say she’s not a Sinn Féiner, she’s a Mé Féiner’ .
2017		https://www.balls.ie 17 Apr. 🌐 We've got a phrase in Ireland – a ‘me feiner’, who is someone who only thinks about themselves.
2021		L. McInerney Rules of Revelation 87: I knew Shakespeare [a gangster] was a mé-féiner.