The former Cistercian College Roscrea schoolboy has won 20 caps to date for Connacht and has played in all but one of Connacht's games this season, scoring four tries in the process.
The 20-year-old also has the honour of scoring Connacht's first ever Heineken Cup try in the narrow defeat away to Harlequins back in November.
O'Halloran has represented Ireland at Under-18 and Under-19 level and went on to play seven times for the Ireland Under-20s, including during last summer's IRB Junior World Championship in Italy.
Michael Swift is Connacht's most-capped player and the only man to have represented the province over 200 times.
Connacht fans will get to enjoy another season of big hits and powerful scrums as the 6ft 5in, 18 stone lock has agreed a year's contract extension.
'Swifty' joined Connacht in 2000 and has played in some of the westerners' biggest games, including the visit of South Africa in 2007, the Amlin Challenge Cup semi-final against Toulon in 2010 and the visit of Toulouse to the Sportsground earlier this season.
Commenting on the contract extensions, Connacht head coach Eric Elwood said: "We're hopeful of having Swifty back this week as we all know what he gives to the team week in, week out.
"Amongst the staff, the management, the supporters he's a legend and this contract is richly deserved, it's fantastic."
O'Halloran admitted: "I am delighted to commit to Connacht for a further two years. Despite some difficult recent results I remain optimistic that this group of players can achieve great things and I want to play my part in that effort.
"Eric Elwood and Nigel Carolan have played major roles in developing my career as a professional player and as a local lad I'm very excited at the prospect of continuing that development here at Connacht."
Today's announcement follows the re-signing of young centre pair Eoin Griffin and David McSharry who have both also committed for a further two years with the side.
"Tiernan has been ever-present this season, he's put himself out there, he's got the contract on merit and he's deserved it," added Elwood.
"The consistency in his performances have been there and he's been one of our top players, so we're delighted to have him sign up for two years."