From A Dictionary of Hiberno-English.
Jim Carroll

Latchiko (colloq., pejor.), an unpleasant, disagreeable person (origin obscure; it has been suggested that the 'latch' suggests children who have to let themselves in by the latch and thus become delinquent as time goes on). 'Who are you calling a latchiko? Watch yourself!'; 'Those latchiko organisations' (Down). Healy, Nineteen Acres, 62: "You looked a bit of a latchiko going to school in Lowpark in hand-me-down American knickerbockers." See FIT-UP; GURRIER.