Green’s Dictionary of Slang

(Irish) an act of masturbation; thus Allied Irish banker n., a wanker n. (2)

1994		Sun. Trib. (Dublin) 27 Nov. n.p.: You turn up, sign a few forms. Then, a quick indulgence in what is known to devotees of rhyming slang as a bit of Allied Irish, and hey presto, there you are 10 quid richer [BS].
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 77: He was probably upstairs having an Allied Irish.
2005		P. Howard Curious Incident of the Dog in the Nightdress 57: Trying to work up the energy to [...] have an Allied Irish. [Ibid.] 199: Hoping that didn’t make me sound like a total Allied Irish Banker.