The BoI would be unsuitable as a modern ent because it is actually quite small inside. Its grandiose exterior belies a relatively small interior. Anyway the old House of Lords / Commons rooms are beautiful, but far too confined.
Remember the Leinster House complex includes many auxiliary offices and has a link to the adjacent Government Buildings. Besides, all the main Governmental offices are around the Kildare St - Merrion St area, so I don't see the Oireachtas wishing to relocate.
I do agree though that it seems a shame that one of the finest buildings in the city belongs to a commercial bank.