The other consideration is the relative strength of the pound against the Euro. When taking families away for the summer, even a £30 difference (which a 99€ vs £99 fare is) will make a difference when 2+ people are travelling. Transport costs for a lot of the market will be relatively similar, but more choice of destinations and frequency from DUB.
3 weeks ago I stayed at Bewleys DUB, I couldn't believe the number of NI registered cars. Easily 30% plus. Maybe more. The islands improved motorway and express coach services to DUB are hurting airports in the South too, ORK is seeing massive leak of catchment to DUB, without APD.
The relative quality of surface transport and the focus that DUB provides will always make DUB more attractive for long-haul and leisure. Removing APD will shift the equilibrium somewhat, but will not move the centre of mass from DUB to NI. This is especially true of flights to North America. CanMericans, which form a much larger possible customer base for NA-Ireland flights, travel to Ireland as a whole. Many don't really have a clue about the Republic vs NI, other than those that have a memory of TV reports on the troubles. Everytime I am over in NI and on the North Cost during the US Summer School holidays the roads are lousy with DUB registered hire cars full of tourists. These trips are often focused on seeing much of the island as possible and not a specific region, it just doesn't really make sense to fly these passengers anywhere other than Dublin. Keep in mind this problem exists in GB, CanMericans travel to London and may venture elsewhere. This significantly skews summer capacity to LHR vs airport like BHM, MAN, NCL, etc. MAN has a much larger catchment population vs DUB, but has far fewer NA flight options.