It marks a meteoric rise for O'Mahony, who has established himself as a key part of Munster's side and has captained the province, as well as winning his first cap during last year's competition.
He started both of Ireland's autumn internationals against South Africa and Argentina and fended off considerable competition from Henry, who has been in impressive form for Ulster.
And O'Mahony is relishing being part of Ireland's future plans, having just signed a new contract with the IRFU lasting until 2015.
"It has been a really enjoyable journey so far for me," he told the Irish Independent.
"The last two seasons, in particular, have been exciting, from breaking into the Munster team and captaining the side and then earning my first cap for Ireland.
"I'm thoroughly enjoying my rugby at the moment and feel there is much more that we can achieve with Munster and Ireland over the next couple of years and I wanted very much to be a part of that."