Dublin South-West TD Paul Murphy is to join People Before Profit (PBP).
Mr Murphy left the Solidarity party in September 2019 to form a new socialist political group after disagreement within the Solidarity/Socialist Party group over its co-operation with other left-leaning parties and left-wing campaigns.
Rise, the group that he formed, remained part of the broader Solidarity-People Before Profit alliance. Rise said in a statement on Sunday it would function "as a network within PBP".
Mr Murphy told the Sunday Independent that the left needed to unite and was well placed to capitalise on the political changes brought about by Covid-19.
“We need to build a broad party of the left which can encompass different threads because politics is changing very rapidly in this country,” he told the paper.