Avoid the hassle of airports and take the ferry from Northern Ireland to the UK.
Guys you're missing something really obvious here.. the border between Eire and Northern Ireland. Have you ever crossed it? Would you know when you had crossed it whilst blasting up the new motorway? Any non EU/UK person who has arrived in the Republic of Ireland can cross into Northern Ireland whenever they want and as many times as they want because there are no Immigration checks done. The risk they take is they may be caught whilst strolling around in Belfast or wherever they end up, no big deal they'll just get a caution and be sent back across the 'border'!
BTW have you never wondered why they have those big mirrors in the entry channels at airports? they're not there for you to admire yourself, guess who sits behind them? Cameras, mirrors they're everywhere.