The Green Party MEP, Ms Nuala Ahern, who has represented Leinster for nearly a decade, will not contest next year's European Parliament election, following medical advice to cut down on her travelling.
"I had a health scare in the spring. I felt better in the summer, but it has been clear from the autumn on that I should not be flying every week," Ms Ahern told The Irish Times.
The four-seat Leinster constituency, which has now been renamed East, has been reduced to a three-seater by the Boundary Review Commission.
The reduction means the constituency, which stretches from Louth to Wexford, has become a much more difficult landscape for smaller political parties.
Ms Ahern dismissed any suggestion that the redrawing of the constituency had had any influence on her decision to step down.
"I was the second person to be elected last time. It is a personal decision. I would love to be challenging next time, and I am very grateful to the Leinster voters. I will have some regrets about not going forward," she said.
The party's deputy leader, Ms Mary White, and a Wicklow councillor, Ms Deirdre de Burca, will seek to replace her when the Green Party holds a selection convention on November 29th.
Ms Ahern said she would be supporting the Carlow-based Ms White, who stood unsuccessfully for the Dáil in 2002. "I would have every confidence in Mary. She is a pragmatist, not an ideologue," she said.
The Leinster MEP has campaigned internationally against the Sellafield nuclear reprocessing plant.
She was one of the first to oppose the opening of Sellafield's thermal oxide reprocessing plant, while she also took part in protests in the Irish Sea against British Nuclear Fuels Ltd transports from Japan.
Last night she said she would encourage the Green Party to support the upcoming European treaty as long as it includes a commitment to review the Euratom Treaty.
"I am going to put it up to the Greens that we should support the constitution on the basis that it makes human rights central. The vast majority of what will be in the treaty has been in previous treaties, and we should support that," she told The Irish Times.
Describing her as "a tenacious and talented campaigner", the leader of the Green Party, Mr Trevor Sargent, said Ms Ahern had been an outstanding representative for Leinster.
"The Green Party regrets but fully understands Nuala's decision not to contest the forthcoming European election. I would like to thank her for her tireless work and achievements over the years," Mr Sargent said.
Cllr de Burca, who is based in Bray, Co Wicklow, said: "I am very interested in Europe. I am disappointed that Nuala is standing down, but it has to be said that an opportunity has been presented."
The Labour Party deputy leader and Wicklow TD, Ms Liz McManus, paid tribute to Ms Ahern, describing her as "a popular and hard-working representative" who had served with distinction in Brussels and Strasbourg.
In 1999 Ms Ahern was elected with 47,184 votes. Fine Gael's MEP, Ms Avril Doyle, will stand again, although the party has yet to decide whether the former Taoiseach, Mr John Bruton, will stand for the European Parliament in the Leinster or Dublin constituencies.
Both of Fianna Fáil's sitting MEPs, Mr Jim Fitzsimons and Mr Liam Hyland, are standing down. The party has not yet selected replacements.