Do moped tyres need the same 1.6 mm tread depth as car tyres in Northern Ireland?

No. The published Northern Ireland vehicle-maintenance guidance says moped tyres must have visible tread.

Car and motorcycle rules are not interchangeable. A moped tyre can still be dangerous if worn, damaged or incorrectly inflated. Check both tyres regularly and replace any tyre showing cuts, bulges or deterioration; visible tread is a legal baseline, not a guarantee of safe grip in wet weather.