The Euros would probably require stadia of bigger capacities RWC. Apart from Dublin where would we get grounds of 40,000+ capacity? If you built a c.40,000 seater in Limerick or Galway that would be overkill for club teams who'd be using the grounds regularly. Also with the expansion of the Euros to 24 teams and the need for 12 host facilities there's no way we'd come up with that many stadia unless we co-hosted it with Scotland (and/or Wales) and ended up providing 6 grounds.
Also with the GAA looking like hosting 8/12 of the RWC games it wouldn't leave much of a sporting legacy. After the tournaments are over the GAA would be left with world class facilities after extensive and costly redevelopment, used exclusively for their own use and rugby/soccer left in the wind with, quite frankly, shite grounds that are barely a step up from the local park.
The Irish RWC bid is a mess in truth and, while I have nothing against the GAA, I would rather have seen greater effort placed in providing for redevelopment of soccer/rugby grounds on the order of ~20,000 in the major population centres that would provide for a sizeable fanbase after tournaments are over. The FAI and IRFU should look towards ground sharing of their facilities around Ireland to keep costs down and providing local teams with world class stadia to provide them with steady gate receipts. So....
Dublin
Croke Park (82,300)
Aviva Stadium (51,700)
RDS Arena (~25,000): Leinster, any Dublin football team
Belfast
Casement Park (~40,000)
Ravenhill (~20,000): Ulster, any Belfast football team
Cork
New Cork municipal stadium (~25,000): Munster, Cork City FC
Limerick
Thomond Park (~25,000): Munster, Limerick FC
Galway
New Galway municipal stadium (~20,000): Connacht, Galway United
Sligo:
Upgraded Showgrounds Stadium (~20,000): Connacht, Sligo Rovers
Waterford
Upgraded WRSC (~20,000): Leinster, Waterford United
Derry
New Derry municipal stadium (~20,000): Ulster, Derry City
One of these? Athlone, Drogheda, Dundalk
That's 12 and a nice spread that provides existing clubs with great new facilities for the future.