Fianna Fáil and Sinn Fein made "happy bedfellows" when they "hounded" former Tánaiste Frances Fitzgerald out of office, the Taoiseach has said.
Leo Varadkar said both parties should "hang their heads in shame" for the way they acted in late 2017 when Ms Fitzgerald was forced to resign amid a controversy surrounding the legal strategy which was used against Sgt Maurice McCabe.
He said that even at this stage they should apologise for what they said because "Dáil privilege is a privilege that should not be abused".
Mr Varadkar was speaking as the former Justice Minister Ms Fitzgerald was chosen to run alongside former SDLP leader Mark Durkan in the European elections.
The two candidates were selected to run for Fine Gael in the Dublin constituency at a meeting in Clontarf Castle on Wednesday night.
Mr Varadkar said "Europe has never been more important" and so it is critical that Ireland is represented by "our best people".
"The European parliament is where things are happening," he said.
Nominating Mr Durkan, Finance Minister Paschal Donohoe said it had fallen on Fine Gael to get the country out of economic crisis and referring to Brexit he said the party will have to "to navigate our way through challenges that are now political in nature".
Outgoing MEP Brian Hayes said over the past 25 years Ms Fitzgerald "showed loyalty to this party at every point along the road".
He said the party would be sending the best team to Europe that represents North and South.
Mr Durkan said he would bring his experience of the Good Friday Agreement and said it should not be treated as "a problem to throw in the way" of Brexit but can be used as a "toolkit" to solve some of the problems.
Ms Fitzgerald this is "a time of huge uncertainty and extraordinary challenge" and said she would represent Ireland's interests in a post-Brexit Europe.
Fine Gael now have seven candidates, four women and three men.