Green’s Dictionary of Slang

(Irish) a sexually attractive/active young person, usu. a woman; also as adj.

1997		Share Slanguage.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Fla/Flah (n): very attractive person.
2006		K. Hoffman Mighty Quinns 16: ‘She was a flah little scrubber all right,’ he said, thickening the Irish accent that still colored the Quinn brothers' voices.