The Cardiff Blues have claimed an automatic place in rugby's Heineken Cup next year after beating last year's winners Munster 20-10.
The Blues' victory leaves the Scarlets and Newport Gwent Dragons to scrap it out in the Magners League's final week to avoid a play-off in Italy.
Tom James and Andy Powell scored first-half tries and Munster's Denis Leamy and Kieran Lewis hit back.
But Sweeney's late interception and Nicky Robinson's conversion ensured Blues victory.
Both teams were much-changed from the games in which they exited the Heineken Cup with the hosts fielding three Lions and Munster two.
The Blues would have sent four Lions into action, but Leigh Halfpenny, who was due to play full back, withdrew due to what was described as a slight knock.
That left Jamie Roberts as the sole Blue Lion behind the scrum with Martyn Williams and Andy Powell up front.
Munster fielded Lions centre Keith Earls and lock Donncha O'Callaghan in a team that was also at half-strength.
