Fianna Fáil MEP for the North and West Seán Ó Neachtain has announced that he will not be standing at the next European Parliament elections in June.
The Galway politician said his decision to step down was the toughest of his life but said he had no option because of health concerns.
Mr Ó Neachtain said his proudest achievement in Europe was overseeing the campaign to make Irish an official working language of the EU.
Taoiseach Brian Cowen said that he fully understood and respected the decision taken by Mr Ó Neachtain not to contest the election.
Mr Cowen added: 'Seán Ó Neachtain is an excellent member of the European Parliament.
'Over the past seven years he has represented the interests and concerns of both his constituents and his country with distinction within the European Parliament.'