The decision was confirmed in a press conference this afternoon, saying it was time to “hand over the leadership reins to the next generation”.
A successor will be appointed at an “early date”, they said.
Shortall said: “As the Social Democrats continue to develop and grow, Catherine and I have decided that the time is right now to hand over the leadership reins of the party to the next generation of Social Democrats.
“We have therefore notified our National Executive that we are stepping down as joint leaders and we have requested that they make arrangements for the election of a new leader and we expect that this will take place at an early date.”
Shortall said it would be left to the party’s other TDs, numbering four in total, to decide on the new party leadership.
The pair insisted that they were not stepping down from their leadership positions due to force. They said the decision had been considered for months.
Shortall was first elected as TD in 1992, representing Dublin North-West, while Murphy was first elected in the period 2005-2007, representing Kildare North. She was elected again in 2011.