Green Party MEP Ms Nuala Ahern has announced she will not stand for election to the European Parliament next year.
Ms Ahern, an MEP for Leinster, told ireland.comshe had taken the decision for personal reasons but that she hoped to continue her work as an environmental campaigner from her "home base".
For the moment, she did not expect to seek political office, although she would remain a member of the Green Party, she said. "I definitely want to continue with my work on the environment in other ways."
Ms Ahern said she was sure the Green Party could retain its seat in Leinster. A selection convention will take place at the end of this month.
However, the Green Party may face an uphill battle without such a high-profile and well-recognised candidate. A recent Boundary Commission report recommended the number of seats in the Leinster and Munster constituencies for the European Parliament elections be reduced from four to three.
The Nice Treaty resulted in the reduction of Ireland's total number of MEPs from 15 to 13.
The MEP said she was sure more than one person would seek the nomination. "We have some very strong candidates in Leinster," she added.
Ms Ahern was first elected to the European Parliament in 1994 and successfully defended her seat in the 1999 elections.
The Green Party leader Mr Trevor Sargent said Ms Ahern has been, and continues to be, "an outstanding Green Party public representative on behalf of the people of Leinster".
He thanked Ms Ahern, on behalf of the party and its members and supporters, for her "tireless work and achievements" over the years. He described her as "a tenacious and talented campaigner".
Ms Ahern was prominent in the campaign against the Sellafield nuclear plant in Cumbria. She is a founder member of the Irish Women's Environmental Network and has also been active in campaigns on the effects of environmental pollution on food and against genetically modified organisms (GMOs).