The Ireland scrum-half appeared to have the world at his feet when he was named in Ian McGeechan's original 37-man Lions squad in April of this year.
Having helped his country to their first Grand Slam since 1948 and played a key role as Munster marched towards the Mganers League crown and the semi-finals of the Heineken Cup, O'Leary looked set to take his fine form on to the world stage when the Lions travelled to South Africa later that summer.
But fate reared its cruel head and O'Leary's Lions dream was over almost as soon as it had begun.
Just days after the highs of learning of his selection for Britain and Ireland's elite, O'Leary suffered the ultimate low in playing terms as injury ruled him out of the 10-match tour.
The 25-year-old suffered a horrific ankle injury when Munster took on the Scarlets five months ago and O'Leary has not played since.
But that extended spell on the sidelines looks set to come to an end this weekend after O'Leary was named among the replacements for, ironically, Munster's clash with the Scarlets in Llanelli.
"Tomas has shown tremendous professionalism and dedication in getting himself back playing and great credit is due to him and to the work done by our medical and fitness staff," said Munster team manager Shaun Payne.
"It is of course great to see him back in the squad particularly after the injury he sustained."
While O'Leary and Alan Quinlan, who was also named in the original Lions touring party but missed out due to suspension, are both included for Munster, the province's actual 2009 Lions all miss out as they continue their rest and re-couperation.
Munster: F Jones, D Howlett, B Murphy, L Mafi, D Hurley, J Manning, T Morland; J Brugnaut, D Varley, J Hayes, D Ryan, M O'Driscoll (captain), A Quinlan, N Ronan, D Leamy
Replacements: D Fogarty, T Buckley, B Holland, T O'Donnell, T O'Leary, S Deasy, T Gleeson