Blues (8) 20
Tries: James, Powell, Sweeney Con: N Robinson Pens: N Robinson
Munster (5) 12
Tries: Leamy, Lewis Con: Manning
Try-scorer Tom James bursts through the Munster defence
Cardiff Blues saw off Munster in a compelling encounter to claim an automatic 2009-10 Heineken Cup place.
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, leaving Chris Czekaj to take over while Tom James came on to the wing.
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.
There was an early Lions injury scare when Andy Powell needed treatment after pulling a maul down on top of himself. But he was soon on his feet.
Nicky Robinson failed with penalty attempts in the fourth and 15th minutes, the latter following Munster's Samoan-born flanker Nick Williams' sin-binning for shoulder-charging Roberts off the ball.
Those failures prompted the Blues to opt for a line-out instead of a shot at goal the next time they had the option.
That paid off with a slick move that ended with James crossing at the corner in the 20th minute only for Robinson's conversion attempt rebounded off an upright.
Munster hardly touched the ball in the opening quarter, Williams' absence hardly helping their cause.
But the visitors' defence was typically determined and for all their efforts the Blues had only James' try to savour.
Having weathered the pressure, Munster mounted an assault of their own, but after also spurning a chance to kick for goal home prop John Yapp gathered a loose pass to help clear the danger.
However, Munster were not to be denied as their battling qualities shone through on the way to Denis Leamy driving over after 30 minutes.
Jamie Roberts' shoulder is treated after on second-half departure
Fly-half Jeremy Manning failed to convert and then had to watch as his opposite number kicked the game's first penalty after Niall Ronan fell offside for the Blues to lead 8-5 at the break.
There was another Lions scare when Roberts departed after the break with what appeared to be a right shoulder injury following a tackle on Peter Stringer.
The immediate diagnosis from the Blues was that the Wales centre was taken off as a precaution and was later seen holding an ice-pack on the affected area.
Martyn Williams denied Barry Murphy a try with a wonderful 61st minute tackle as Munster continued to press and probe.
The visitors' reward came two minutes later, wing Kieran Lewis crossing at the corner and Manning converted to give them the lead for the first time.
Dai Young's side were not finished, fighting their way back down field for replacement hooker Rhys Thomas's pass to send prop Gary Powell over.
Fly-half Robinson failed to convert and after Mick O'Driscoll fell offside, fluffed another kickable penalty attempt.
The Blues' nerves were settled when replacement Ceri Sweeney intercepted a Manning pass to race over unopposed and Robinson's conversion ensured there was no way back for Munster.
Cardiff Blues boss Dai Young:
"We've enjoyed the Heineken Cup this season and wanted to make sure we're there next season and to do it with two games in hand is pleasing.
"Large parts of our game were a typical end-of-season game, a bit sloppy, our execution and skills weren't great out there.
"We're at a stage now where there's a lot of tired bodies, both mentally and physically and I think everyone just wants to get over that last hurdle of the last couple of games."
Cardiff Blues: Czekaj, Mustoe, J Robinson, Roberts, James, N Robinson, Allinson, Yapp, G Williams, G Powell, Davies, Morgan, Warburton, M Williams (capt), A. Powell.
Replacements: Sweeney for J. Roberts (46), R Thomas for G Williams (70), D Jones for Morgan (56), White for A Powell (70).
Not Used: Sowden-Taylor, Rees, Hobbs.
Munster: D Hurley, Lewis, Murphy, Earls, Dowling, Manning, Stringer, Pucciariello, Fogarty, Hayes, O'Callaghan, O'Driscoll, N Williams, Ronan, Leamy.
Replacements: Burke for Manning (78), Buckley for Pucciariello (41), Essex for Fogarty (76), Foley for N Williams (77).
Not Used: Coughlan, Murray, J. Holland.
Ref: Peter Allan (SRU).