The 30-year-old former Wigan favourite returned to his native Cork to resurrect his career two months after dropping his bombshell announcement about retirement.
Munster confirmed his capture on a two-year contract just before the transfer deadline and immediately added him to their squad for the knockout stages of the Heineken Cup.
The deal went through after Munster arranged a compensation deal with Australian rugby league club Gold Coast, with whom Carney had agreed a two-year deal.
"It's really a tremendous honour to be signed up by a club like Munster," Carney told the club's website, www.munsterrugby.ie.
"My first objective is to get into that Munster side and become a player that the players want in the team.
"To do that I know I have a lot of hard work ahead of me. But I'm prepared to knuckle down and see how things progress.
"Knowing the history of the club, it is a huge honour and I hope I can become a part of a winning successful side."
The autumn World Cup is an obvious incentive for Carney, who is expected to be fast-tracked onto Ireland's summer tour to Argentina but has precious little time to adapt to a game he last played regularly at the same school as centre Gordon D'Arcy.
Munster coach Declan Kidney said: "Given the injury situation in relation to Shaun Payne, Mossie Lawlor and Anthony Horgan, I think it's particularly opportune to acquire someone of Brian's stature and experience for the end-of-season matches in Magners League and Heineken Cup."
The one-time Gaelic footballer was spotted playing rugby league for a Dublin students team by Gateshead in 1999 and went on to play for Hull and Wigan before moving to Australian club Newcastle Knights last season.
He played for Ireland in the 2000 World Cup and won the last of his 14 Great Britain caps in their 23-12 win over Australia in Sydney in November.