Green’s Dictionary of Slang

flah v.

(Irish) to have sexual intercourse.

1996		(con. 1970) G. Moxley Danti-Dan in McGuinness Dazzling Dark (1996) I iv: Yeah, we’re saving ... then I can flah you cross-eyed whenever I like.
1997		Share Slanguage.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Fla/Flah (v): to have sexual intercourse with someone ( From Irish ‘Fleadh’ meaning party).
2017		L. McInerney Blood Miracles 129: ‘I had to leave the chickie I was all set to be flahing tonight’.
2021		L. McInerney Rules of Revelation 156: Were you flahing her, too? That Izzy wan? Is that her problem with you?’ ‘I was not fucking flahing her,’ he spat.

In phrases

flahed out (adj.)

(Irish) exhausted, fucked adj.¹ (2)

2017

L. McInerney Blood Miracles 98: ‘You’re not up for the party so.’ ‘I’m flahed out from partying’.