BTW, Markie, it is not just in remote rural areas: you can cross the border freely anywhere as there are no routine border controls (the infrastructure of border posts is not even there anymore.
It is not a question of particular roads. Going from the Republic to NI there isn't, nor has ever been, any immigration control or even a way to sort immigration issues. So the Dublin airport immigration have to anticipate any problems. Apart from the advice to sort things out, in the very short term the OP might find it simplest to visit a Schengen country and meet her boyfriend in Paris or Amsterdam.