The Anti Austerity Alliance and People Before Profit have formally launched their new electoral block.
They are now officially on the Register of Political Parties in the hope that together they can get at least seven TDs elected and give them greater rights in the Dáil.
However, they will remain as two separate organisations, though together they hope to run at least 30 candidates in 27 constituencies.
Deputy Richard Boyd Barrett says they want to get a new voice for the left.
He said: "This is, I think, the biggest intervention by the forces of the left, to the left of the Labour Party, probably in the history of the State.
"It comes at a time when there is a greater thirst and demand by huge numbers of ordinary people in this country for a radical break with the political establishment."