Clubs may register up to two additional players (one being front row cover) during the pool stages, each to replace a player previously registered.
O'Donnell was left out of the original squad after suffering a hyper-extended left knee injury against Zebre in September but played 32 minutes for Munster A against Plymouth on Friday and came through with no issues.
Ronan is continuing his rehabilitation from a sprained knee and could be back training next week.
Elsewhere on the injury front, Ian Keatley is expected to return to training tomorrow after picking up a dead leg during the weekend's game against Perpignan. On the injury, Rob Penney said yesterday: "It's still touch and go but it's probably more likely than not that he should be fit, we won't know until he starts running again at the end of the week."
Donnacha Ryan (strained knee ligaments) has been out since the Gloucester game in October but took part in training last Friday and has been progressing well.
Conor Murray suffered damage to the medial ligament of his right knee at the weekend and has been ruled out for up to six weeks.
Luke O'Dea broke the thumb in his right hand playing for Munster A against Plymouth Albion on Friday night. O'Dea had surgery at the weekend and has been ruled out for approximately eight weeks.
Mike Sherry will undergo surgery on December 19th after rupturing the ACL in his left knee against the Dragons and is expected to be ruled out for up to six months.
Andrew Conway suffered a blow to his knee in the Cardiff game and has yet to return to training as the knee is still bruised and swollen.
Gerhard van den Heever injured his right hand against the Dragons but it's not as bad as first expected and the South African will return to training over the Christmas period.
Paddy Butler could return to training tomorrow as he continues his recovery from a fractured right thumb that he sustained against Edinburgh in October.
Simon Zebo's recovery from a cracked bone in his left foot is progressing well and the winger should return to training over the Christmas period.