Having made his Munster debut against Leinster in August, 1999, Horan came on as a replacement for the first of his 84 Heineken Cup appearances against Pontypridd at Thomond Park on 20 November. Over 14 seasons he has become
Munster's most capped forward with 224 appearances and he also won 67 Irish caps - including a Grand Slam in 2009.
His first Heineken Cup start came three weeks later when he propped up a front row that also contained Keith Wood and John Hayes as Munster recorded their first win on French soil with a win over Colomiers at Stade les Sept Derniers. Munster went on to reach their first final that season and Horan figured in the quarter-final and semi-final victories over Stade Francais Paris and Toulouse.
After announcing his retirement details to his Munster team mates at training, Horan got back down to business ahead of this weekend's vital RaboDirect PRO12 clash with arch-rivals Leinster.
"It was a tough decision to make, no question of that, but I'm happy given the options presented to me. It was the right one for me and my family," said Horan.
"I've enjoyed my career no end and I'm proud of what I've achieved. To that end I'd like to thank all those involved for helping me along the way be it at club provincial or international level.
"But I couldn't have achieved what I did without the support of my family and I am grateful to them beyond words. I'm looking forward now to being able to spend a bit more time with them.
"Talking about support, I have to mention the marvelous Munster supporters whose unwavering loyalty down the years was another huge factor in making my career so memorable. Fabulous people."
Horan has made three appearances for Munster this season, as a replacement in the Pool stages, and will be hoping to see his team help him sign off in style with another Heineken Cup final occasion next month.
With 84 Heineken Cup appearances to his name Horan is the seventh most capped Munster player in the tournament and currently locked in 10th place with Toulouse' Jean Bouilhou on the all-time appearance list.
Munster team manager, Nial O'Donovan, was the Munster forwards coach when Horan first came into the squad and saw him develop into one of his country's greatest forwards.
"I've known and been involved with Marcus since he was an 18-year-old starting out with Shannon and subsequently with Munster and Ireland as well. Throughout that time he conducted himself with dedication in a truly professional manner and in a Munster sense his contribution has been immense," said O'Donovan.
"He's been a great team player, an exemplary squad member, an example to others. I understand how difficult it must be for Marcus to make this decision, it's never an easy one, and it presents a challenge.
"But I've no doubt he'll meet that challenge with the same courage he showed on countless occasion in the past both on and off the field. "