sorry to hear that Kiely died ,thanks for all the info . I'm inclined to think that the "latch key" origin is the most likely.Whatever way it's spelled, the "k" sound is definitely there in Lacheko .

Honoria's description of a latchico is about right- he isn't the sort you'd tend to say "Latcho drom" to if you met him in a pub, Lavengro . If you told a latchiko to have a safe journey home he'd probably put the tap on you for the cab fare.