Green’s Dictionary of Slang

(Irish) a little man, a gauche young man.

1916		Joyce Portrait of the Artist as a Young Man 103: I was standing at the end of the South Terrace one day with some maneens like myself and sure we thought we were grand fellows because we had pipes stuck in the corners of our mouths.
1943		‘Myles na gCopaleen’ Faustus Kelly in ‘Flann O’Brien’ Stories & Plays (1973) 128: He was not a smart maneen from Cork with his degrees and all his orders.
1965		L. Dunne Goodbye to The Hill (1966) 113: ‘He’s nothing to shave,’ Billy snorted. ‘He’s only a maneen, trying to be big.’.