The sponsorship begins in time for the Triple Crown-winning team's New Zealand and Australia summer tour, which begins on June 10, and will continue up to the 2011/2012 season. The telecommunications company takes over from Permanent TSB bank.
As part of the agreement, O2 Ireland will have the rights to place its logo on all Ireland senior international and Ireland A team playing and training kit and team-related IRFU merchandise.
The partnership will also give O2 Ireland key sites on advertising hoardings surrounding the pitch at home matches, which will be played in Dublin's Croke Park while Lansdowne Road is being upgraded.
Andy Crawford, IRFU president, said: "We are proud to further consolidate our partnership with this new agreement, and look forward to enjoying many successful years ahead with O2 and the Irish team."
Danuta Gray, chief executive of O2 Ireland, said that Ireland's winning of the Triple Crown and second place in the Six Nations performances enhanced the sponsorship deal. She said: "Ireland's tremendous performance in the Triple Crown and its impressive performances in the Six Nations is testament to the skill of today's players and the successful structures put in place by the IRFU."
As part of the deal, O2 Ireland customers will have exclusive access to a selection of interactive services on i-mode such as player diaries and text alerts with the latest news and scores.
They will also be offered a preferential rate for membership of the Ireland Rugby Supporters Club and O2 Ireland will introduce a specially commissioned Ireland supporters' handset.
Meanwhile, one of Scotland's three professional sides, including Edinburgh Gunners, Glasgow Warriors and Border Reivers, could still face closure due to a lack of sponsorship, the Scottish Rugby Union has said.
The SRU is understood to have received offers of investment from 11 different groups, but only four were realistic proposals.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .