Incidentally, the "throwaway city" trick used to be very common. Years ago, it was $300 cheaper to fly from ORD - DUB - SNN than to go ORD - DUB. I did exactly as you described (just de-planed at DUB and then boarded at DUB on the return), and it was fine. Nowadays, the airlines have pretty much wised up and will cancel the remainder of the itinerary if one skips any segments.