An early try from Lions veteran Shane Williams set Wales on their way when the Ospreys wing squeezed in at the corner following patient phase play and a well-timed pass from Leigh Halfpenny.
Rhys Priestland's well-struck touchline conversion handed Wales a seven-point lead against an Irish outfit who started the quarter-final as narrow favourites following their stunning success over Australia in the group stages.
Ireland dominated possession for much of the half but Wales defended brilliantly, with lock Luke Charteris making an incredible 19 tackles.
Ronan O'Gara cut the gap to four points with a simple penalty but Ireland failed to cross the Welsh line having twice turned down kicks at goal in favour of close-range lineouts.
A long-range penalty from Leigh Halfpenny re-established the seven-point advantage and thing stayed that way until half-time.
Whereas Wales enjoyed a great start to the first period, Ireland had the perfect beginning to the second 40. Keith Earls, yet another of the 17 Lions on show, slid in at the corner despite the attentions of Mike Phillips and O'Gara's conversion from wide on the left levelled the scores at 10-apiece just three minutes in.
But Wales hit back and dominated the majority of the remainder of the match, especially after Phillips spotted the narrowest of gaps for a fine individual try on 51 minutes.
Jonathan Davies added a third try 10 minutes later, with Priestland adding the extras in between striking separate posts with two penalty attempts.
Ireland looked to bounce back with a quick score of their own but the Welsh defence was nothing short of incredible as their daunting training camps in Poland paid dividends.
Wales can now look forward to a semi-final showdown with England or France next week while Ireland will head for home tomorrow with the likes of Lions legends Brian O'Driscoll, Paul O'Connell and O'Gara looking like they've played their final World Cup game.