The 21-year-old signed the deal upon returning from international duty with the Republic of Ireland Under-21's to end any hint of a Griffin Park exit.
"There have been a lot of players released for financial reasons," the forward told the club's official website.
"I'm just happy to sign, get a bit of security and play at the club I'm happy with.
"Being able to start last season, I went from strength to strength and hopefully this season I'll be able to get better and better with a bit of luck."
Meanwhile, The Bees now look certain to lose highly-rated winger Steve Hunt to Second Division rivals Brighton & Hove Albion.
The 21-year-old was brought to Griffin Park by former boss Steve Coppell, now in charge at Brighton, and Hunt looks set to reject Brentford's final offer for a more lucrative deal at the Withdean Stadium.
Also on his way out of Griffin Park is midfielder Martin Rowlands who looks set to join London neighbours QPR.
Rowlands is still recovering from broken leg, but is set to clinch a move to Loftus Road once he has regained full fitness.
