Dublin South West TD Paul Murphy has established a new political grouping called RISE - Radical Internationalist Socialist Environmentalist - following his resignation from the Socialist Party last week.
Mr Murphy, who is RISE's only elected member, said his grouping would promote what is termed a 'Green New Deal' of socialist policies for Ireland, which would be published shortly.
Speaking at the launch in Dublin this afternoon, Mr Murphy said he would remain part of the Solidarity-People Before Profit group in the Dáil, even though he was no longer a member of the Socialist Party.
He added that RISE would seek to work with other left-leaning parties, such as Sinn Féin and the Greens, as well as broader left-wing campaigns to further socialist objectives.
The environment, and climate change in particular, are due to be key concerns for the new grouping.
Mr Murphy said that science indicated that urgent radical reform was required which encompassed all elements of the economy from transport to employment to energy and agriculture.
He said that climate science dictated Ireland needed to have a zero-carbon economy by 2030 - including what he called "free, green and frequent" public transport.
The Dublin South West TD acknowledged that a further fracturing of left-wing politics would be frustrating for some people, but he replied: "Watch what we say, watch what we do, let things play out over a period of months. I think the differences in strategy will become clear to people."
Mr Murphy said the electoral ambitions of RISE were modest and the grouping would not attempt to run "a raft of candidates" in the next general election.
That said, the deputy indicated RISE would be in favour of leftist parties agreeing on candidates in order to maximise their chances of success.
He dismissed a suggestion that RISE would be viewed as "the Paul Murphy party".