Horan earned his first senior competitive cap in the Interprovincial series in August 1999 against Leinster and the first of his 84 Heineken Cup caps came later that season when he came on as a replacement for Peter Clohessy in Munster's 32-10 win over Pontypridd at Thomond Park.
His first Heineken Cup start came three weeks later in the exalted front company of Keith Wood and John Hayes in the historic first ever victory on French soil over Colomiers at Stade les Sept Derniers, an indication of his burgeoning talent, his comfort at playing the second half of that game at tight-head after Hayes retired injured.
He also deputised at tight-head in the subsequent games, the return against Pontypridd; after 23 minutes of the quarter-final against Stade Francais and again in the famous semi-final win over mighty Toulouse in Bordeaux.
He continued to serve his time to Clohessy, sitting on the bench behind him in the 2000 and 2002 finals but when the 2002/03 season kicked off Horan was wearing the number 1 shirt and made it almost exclusively his own for the ensuing seven seasons in which time he played a crucial role in Munster's Heineken Cup success of 2006 & 2008 as well as Celtic League glory in 2003 and Magners League in 2010.
He won his first 6 Nations cap in season 2002/03 and was the starting loose-head in all of Ireland's games when they claimed the Grand Slam in 2009.
Acknowledged and respected as one of the finest loose-heads of his generation, he more than held his own in that most demanding department and always against the very best of compan, his football skills sees him remain one of Munster's leading try scorers with 11 in the Heineken Cup out of a career total of 30.
Niall O'Donovan was Munster forwards coach when Horan started and ironically is the current Team Manager as Horan calls time and he had this to say. â00" 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. â00
"Throughout that time he conducted himself always with dedication, always in a truly professional manner and in a Munster sense his contribution has been immense.
"He's been a great team player, an exemplary squad member, an example to others.
" â00I understand how difficult it must be for Marcus to make this decision." O'Donovan continued, " It's never an easy one, and it presents a challenge. But I've no doubt Marcus will meet that challenge with the same courage he showed on countless occasion in the past both on and off the field."
As for the player himself, it was business as usual. After breaking the news to colleagues, he trained with the squad in CIT, afterwards saying, â00" Course it was a tough decision to make. No question of that. You don't walk away from a group like this easily or without sadness. They've been such a huge part of my life.
"But I'm happy giving the options presented to me, it was the right decision for me and my family. â00 I've had a great career, I've enjoyed every minute of it 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.
"â00But 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. Hopefully give a little bit back to them now"
And he concluded by saying. " â00Talking about support, I have to mention the marvellous Munster supporters whose unwavering loyalty down the years was another huge factor in making my career so memorable. Fabulous people.â0"