Mr Caoimhghin O Caolain has been named Sinn Fein's leader of the parliamentary party in the Dail.
The 49-year-old made history in 1997 when he headed the poll in the Cavan/Monaghan constituency to become the first Sinn Fein representative in 40 years to take a seat in the Dail.
When the lower house meets in June - for the first time since the General Election - he will head a five-strong team.
Mr O Caolain was chosen at a special meeting in Dublin today of Sinn Fein's Ard Chomhairle.
After the announcement of his appointment, Mr O Caolain pledged "constructive and critical opposition" in the Dail. "With a mandate from almost 300,000 voters, we are also part of a larger national Sinn Fein team, committed to bringing about real change on an all-Ireland basis," he said.
"We are seeing the beginning of a realignment in Irish politics. Sinn Fein's only interest in increasing our political strength is to utilise that strength to bring about equality, to end poverty, and factor equity and justice into the rights of citizens in their entitlements to decent jobs, a public health service, education and housing."
The party’s chief whip will be Mr Aengus O Snodaigh.
After today's meeting, Sinn Fein president Mr Gerry Adams demanded a "rigorous review" of progress on implementing the Belfast Agreement.
He said there had to be a stock-taking that included the record of both the British and Irish governments on delivering their obligations on the 1998 accord.
Mr Adams said: "The British government, in particular, has a case to answer in relation to its failure to deliver a new, civic accountable policing service, its lack of progress on the crucial issue of demilitarisation, the serious gap that exists in the promotion of the equality agenda, or the creation of a bill of rights, and much more."