Being Welsh I think Wales will win But it's going to be a very close game.
Ireland have got a lot of experienced players and it's back row is working very well together but out of all the teams in the world cup I think Wales have played the most dynamic rugby and I think it's the first time I have seen Wales play the full 80mins without running out of steam.
Wales's strong point are their backs who can score tries from anywhere on the field. Their only slight weak point is the forwards, they don't seem to have the natural aggression to go out and rough their opponents up and force them in to mistakes and turn over ball. They also need to do a bit of work on their break down as they need to clean the opposition out of the way quicker to allow the scrum half to get the backs moving quickly to exploit the holes in the oppositions defence.
If you look at the stats Wales have played the 2nd, 10th, 16th & 19th IRB ranked teams in the pool stage, Ireland have played the 3rd, 11th, 17th & 21st, so not much difference there, but on paper Wales have played the better teams.
At the group stage Wales scored 180 points and conceeded 34 points, Ireland score 135 and conceeded 34 points. So the defences are evenly matched but Wales has the advantage in attack. Also Wales had the second highest points score and 2nd highest try count in the group stages only New Zealand having higher.
Wales to win by 1 to 3 points
If England beat France then I think Wales or Ireland will get to the final, but France are a bit of an unknown as they seem to change most of their players from game to game so who knows what French side will turn up.