I would say that's a no. Without the exact details I would have to make some assumptions here, but in essence you splitting off DUB-LHR-DUB turns the ticket into 2 trips, and that won't get through the computer system, never mind the fare basis. To answer the general question, yes there are a lot of restrictions in this area, but if you bought a ticket from Dublin to say Denver, it either has to stay as that, or be restructured within policy.