"Harmless," in Irish English, means "dim-witted" or "simple." It can even be used in description of somebody who is dangerous.
I did a bit of work with a Kilkenny friend at a residential facility operated by the Southeastern Health Board in the care of a young developmentally-disabled man. Before the first time we went out there, the maintenance man for SEHB warned Gary "don't turn your back on him," and not to leave any dangerous implements about. The lad would be likely to take anything weapon-like and hide it for later potential use.
On one occasion, the young fellow had broken out every window at the facility and there are a lot of windows.
But he's a simple fellow. Late in his teens, he has a collection of toy cars that one would associate with an 8-year-old. He's friendly, and curious about the work we do cleaning a chimney on one occasion, shampooing the carpet and furniture on another (after he'd peed in a corner.)
In Hiberno-English, he's "harmless." Dangerous but kind of simple.