Green’s Dictionary of Slang

(Irish) a mortal sin, as set down by Roman Catholicism.

1983		E. Mac Thomas Me Jewel and Darlin’ Dublin 39: I don't think anyone in the world can tell what a mortaller (mortal sin) looks like.
1996		Sun. Trib. (Dublin) 24 Mar. n.p.: Oh, shaggin’ hell, I’m nearly after denting the Beamer, trying to steer past that oul’ one in her clapped-out Lada. Oh God, I suppose that’s two or three more mortallers I’ve clocked up.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Mortaller (n): mortal sin.
2005		D. Moore Diary of a Man 189: In 1975, ‘Persona Humana – Declaration on Certain Questions Concerning Sexual Ethics’ was issued by Paul VI, and in it [...] masturbation was deemed a ‘grave moral disorder’. A ‘mortaller’ .