The proposal was actually frowned on in a recent report (co-authored by SH&E, the well known aviaiton consultants). However, the current airport site is the only game in town, Baldonnell (the military airfield) being pretty much dead as an option.
There will be complications and as the Guv'nor rightly said, Aer Rianta can be expected to make life difficult. However, that said, they can only pose delays and obstructions.
There is a SORE need for competition in the airports sector and from a national economic point of view, you will end up with two well established operators (I don't give much for the proposed Huntstown project - there will need to be an international heavy-hitter: SATS, BAA, Aeroport de Paris, etc.) competing aggressively to market Dublin.
However, even before this, the very threat of competition will put the skids under Aer Rianta, in that operators will know they can hold the threat of moving to the new terminal over it for a long time before it opens.