Cardiff - The Republic of Ireland claimed a World Cup play-off spot at Wales' expense after James McClean's crisp second-half strike earned his side a 1-0 victory in Cardiff on Monday.
The West Bromwich Albion winger's goal moved Ireland above Wales into second place in Group D below Serbia, meaning Martin O'Neill's men will be one of the eight teams in the European play-off draw on October 17.
"The performance of the players was absolutely fantastic," said Ireland manager O'Neill.
"We had to withstand a lot of pressure, which you would expect. But we had to win tonight in Cardiff and we've done exactly that.
"James scores a great goal for us and then we have to withstand more pressure. Some great, great performances all round. And so we go to the play-offs."
While Ireland remain on course for a first World Cup finals appearance since 2002, surprise Euro 2016 semi-finalists Wales saw their dream of a first appearance at the tournament since 1958 turn to dust.
Bereft of Real Madrid star Gareth Bale, and robbed of Joe Allen by a suspected concussion in the first half, Wales fell to their first defeat in the group and their first loss in a competitive home game in over four years.
The tense game at Cardiff City Stadium stands to be Chris Coleman's last as Wales manager in a competitive fixture, the architect of last year's Euro heroics having pledged to stand down after the current World Cup cycle.
"We just came up a bit short tonight," Coleman said. "Our imagination, offensively, we didn't quite have enough.
"I can't say right now about my future because I'm not thinking about it. The dressing room's full of devastated players and staff.
"The whole nation will be mourning because that elusive World Cup has passed us by again."
Brought into the Wales team in place of Sam Vokes, Hal Robson-Kanu went close early on with a header from a Joe Ledley corner, before Aaron Ramsey saw a crisp drive tipped over by Ireland goalkeeper Darren Randolph.
Wales dominated possession in the first half, creating concern among the green hordes inside the stadium and obliging Ireland to feed off scraps in the final third.
A chance fell to Jeff Hendrick when Cyrus Christie's long throw-in was not cleared, but the Burnley man miscued, and Shane Duffy drilled a low shot across the face of the Welsh goal.
Allen and Ramsey were setting the tempo for Wales with their neat footwork and incisive passing, only for the Stoke City midfielder to be forced off in the 37th minute after being caught in a sandwich between McClean and David Meyler.
Jonny Williams came on and the loss of one of Wales' key orchestrators enabled Ireland to gain a foothold for the first time.
Wales started the second half brightly and it took a brilliant acrobatic save from Randolph to prevent Robson-Kanu putting them in front with a header from Jonny Williams's right-wing cross.
But in the 57th minute Ireland struck, Hendrick charging down Ashley Williams's clearance and crossing from the right, Harry Arter dummying and McClean slamming a right-foot shot past a motionless Wayne Hennessey.
It was the first time Wales had fallen behind in the qualifying campaign.
Coleman sent on 17-year-old Liverpool forward Ben Woodburn and Vokes, but Ireland survived heavy late pressure to extend their unbeaten record against their Celtic cousins to eight games and stay in the hunt for Russia.