Welsh rugby was already rocking from the sudden departure of Grand Slam-winning head coach Mike Ruddock amid rumours of player power being a major factor.
And now their grip on the title is effectively over with Ireland, Scotland and England tied on four points apiece.
Wales, without injured captain Gareth Thomas, made a dominant start inspired by fly-half Stephen Jones, and they took an eighth-minute lead through winger Mark Jones.
But Stephen Jones limped off injured after 19 minutes and that proved the turning point.
Gavin Henson, returning to Test rugby for the first time in a year, endured a miserable afternoon in his place and Wales lost all momentum.
Wallace charged over from the back of a scrum and with O'Gara landing a penalty, Ireland led 8-5 and their tails were up.
Ireland were immediately on the front foot after the restart. Brian O'Driscoll's break forced Dwayne Peel to clear up but soon afterwards Horgan found a giant gap in the Welsh defence and ran over untouched for Ireland's second try.
O'Gara converted and then landed a third penalty as Ireland raced clear.
Marcus Horan was taken off on a stretcher with his neck in a brace after being caught awkwardly as he attempted to secure a loose ball.
But Ireland finished with a flourish and scrum-half Stringer dived over from close range to complete the scoring.