Steviec9
If you arrive from Ireland (ROI) you are an International arrival not Domestic, hence no need for the photograph on arrival. When you get to Passport Control, because Ireland is part of the Common Travel Area, there is no need for your passport to be checked, hence the separate arrivals corridor at Passport Control. UK Domestic arrivals come through a different channel and collect their baggage from a different area. This involves them mixing with departing passengers, hence the photos are taken on arrival to ensure only genuine domestic arrivals are able to exit through the domestic channel.