The photo ID/ Landing card check provides a means to ensure that all arrivals do in fact pass from the aircraft to landside. Any swap of Info would show up at the check where the operator can reconcile the bar code with a photo. Missing pax or wrong ID's will show up.
At LGW South, there is NO Photo ID check for ROI arriving pax, only UK domestic. Accept that there might be some later reconciliation problems with a spurious boarding card but
there is NO Photo Check during ROI arrivals, you
do NOT show a passport or any form of ID and there
is NO computerised check of boarding cards during ROI arrivals, you just shove your boarding card stub in the fella's hand and walk off. By the time any later (manual) reconcillation has taken place, miscreant is well away from airport.
My original loophole method above still stands - it IS possible. I accept all that everyone here has said regarding the various controls, hidden police presence etc. It does not alter the fact that there are no actual, real time, identity checks carried out during the ROI arrivals process as there are for other (non CTA) international arrivals and even UK domestic arrivals. On the reverse journey, the Gardai check ID of all incoming passengers (certainly at DUB and SNN, which I use), regardless of origin. They accept a UK Driving Licence but have even said 'we just need to make sure who you are'.