He moved to Rotherham via Connacht where he caught Gloucester's attention, scoring 11 tries for the Titans in 2011-12.
The 25 year old made a promising start to life at Gloucester as he played his part in Gloucester's JP Morgan Asset Management Premiership Rugby 7s campaign but is now itching to show what he can do in the 15s version of the game.
And, returning to the club where it all started for him, affords Monahan the perfect opportunity to show what he's capable of, something he feels he didn't have the chance to do at the time.
"I never really had a proper opportunity to show what I could do. If I'd played and wasn't up to the standard, then fair enough.
"But I always knew I was capable of playing this standard, and I wanted to prove that to myself more than anything. That's a big reason why I took that Rotherham chance with both hands.
"Thankfully it's worked out so far, but I've got to keep that desire, and keep working hard to earn my chances."
"It'll be my first time playing rugby in Leinster anyway. I played from Under-16s to Under-20s level, I played a lot of A games but only had one cap with the senior team.
"I had wanted to play for Leinster but that didn't happen for various reasons, so I had to go elsewhere to get my opportunity to play.
"The fact that I'm coming back to Leinster playing with Gloucester, it's fantastic, to come back with this calibre of team. And I can't wait for it."