Ms McManus, 63, said it was vital that Labour offered “new, younger candidates” at the election, which must take place by the summer of 2012.
“It has been a privilege for me to represent Co Wicklow for the Labour Party and I will continue to do so to the best of my ability until the next general election,” she said.
“I believe, however, that it is vital that Labour offers new, younger candidates to the electorate at the next general election.
“Our determination must be to win two seats in Co Wicklow. With the right candidates and a strong campaign we can achieve that goal.”
Ms McManus has been a public representative for 31 years, first as a councillor before being elected to the Dáil as a Democratic Left TD in 1992.
She went on to on to serve as a junior minister in the Fine Gael-Labour-DL “rainbow coalition” of 1994-1997.
DL merged with Labour in the late 1990s, and Ms McManus become deputy leader of Labour in 2002.
She stepped down from that position after the 2007 general election, but remained on Labour’s frontbench as its spokeswoman on communications.
Mr Gilmore said Ms McManus had advised him “some time ago” that it was her intention to step down at the next election.
“Liz will be a great loss to the people of Wicklow, to the Labour Party and to the Dáil, but I fully respect the decision she has made,” he said.
“Liz is a person of great personal and political courage, always willing to take on vested interests and who was never afraid to take a principled stand on matters of importance to her.”
Ms McManus holds Labour’s only seat in Wicklow. Labour has six members on Wicklow County Council who may seek to contest the election, including Cllr Nicky Kelly, who ran in the 2007 general election but failed to win a seat.