The Irish went into the game at the Perth Rectangular Stadium knowing they could not make it to the knockout stages after Papua New Guinea had earlier destroyed the USA 64-0 to ensure they would take spot in Group C.
Therefore, Papua New Guinea's previous 14-6 win against the Irish proved to crucial in determining which of the two sides would progress into the quarter-finals.
Nevertheless, Mark Aston's side shrugged off their disappointment and, having already defeated Italy 36-12 at the tournament, they dished out a comprehensive beating to the Welsh to go out in style.
Api Pewhairangi, a brace from Oliver Roberts, Joe Philbin, Liam Finn and Liam Kay all crossed for the Irish, while Wales' consolation try came from Ben Morris just after half-time.
It was a third successive heavy defeat for Wales and they have now failed to win a World Cup match since 2000, having also lost their three group matches in 2013.