Tierney will return on a full-time basis at a time where Welsh rugby finds itself in yet more crisis.
Steve Tandy’s men’s national side have just suffered a record 73-0 home defeat against South Africa, while Wales captains Jac Morgan and Dewi Lake have been linked with a move from Ospreys to Gloucester.
The WRU announced in October that it planned to reduce the number of professional men’s sides in Wales from four to three.
Welsh rugby’s governing body confirmed it proposes to grant three licences for men’s clubs.
There will be one in Cardiff, one in the east and one in the west, which is expected to result in a straight survival fight between Ospreys and Scarlets in Llanelli.
Another option that has now emerged is Ospreys owners, Y11 Sport & Media, taking over WRU-owned Cardiff, which could produce the desired number of three professional sides.
“Of course, there are significant and major challenges presented to Welsh rugby at this time,” said Tierney, “but there is also a huge opportunity for us to forge a way ahead in our professional game which will safeguard its future.
“There is a lot more hard work ahead, but our strength in Welsh rugby is in our ability to unite and work together towards one collective aim.
“I am looking forward to contributing to this again.”
Collier-Keywood will revert back to his responsibilities as chair.
“I know I speak on behalf of the whole game in Wales when I welcome Abi back into her role,” said Collier-Keywood.
“We are making progress in our transformation programme for the professional game in Wales, but Abi’s return will add new and positive impetus at this pivotal time.”
