Celtic current play out of Brigend, which is about a 20-30 minute train ride and walk from Cardiff, about 30-40 minute train ride/walk from Swansea (it sits roughly inbetween the two cities).
I'm not aware of their specific plans, but I'd imagine in the short term they would continue this arrangement as the ground is not great but it does the job for their crowd level and game day there seems a fun experience for all (cider!). And they'd risk losing some of their existing foundation supporter base if they shifted to one of the cities too quickly. On top of MM, they'd probably look for an opportunity to play the occasional game somewhere in Cardiff to raise profile, maybe home game v Harlequins closer to Cardiff might make sense, depending on cost of ground and accessibility to the train route?
Croke Park would be great in Dublin, I just don't know how many current fans would fly, and how many potential league fans there would be, and what is to be gained out of raising interest there if there are no longer term plans? You'd lose the benefit of raising interest in Wales or France, as you say.
I'm not sure jumping right into Paris would be the best move, Catalans have got a few headlines in French national papers from their challenge cup run last year, but it might be better off holding MM in Toulouse once they've finished with Cardiff, since there is an intention/bid for a superleague team, and building up momentum and profile in France from there?
If we're talking about 30-40,000 fans max, I think then there are grounds in Toulouse that can do that. Cardiff 2009 (if Celtic Crusaders get in), Toulouse 2010-2012 (if Toulouse miss out, or get in and need a second year boost)?