Fianna Fail has moved to distance itself from Eurosceptics in Brussels after its only MEP defected to their ranks.
Brian Crowley, elected to represent Ireland South in last month's European elections, sparked fury among colleagues after joining the European Conservative and Reformists Group (ECR).
The move aligns him with right-wingers such as the British Conservatives and some far-right wing groups in the European Parliament.
Indignant Fianna Fail chiefs have lambasted the manoeuvre, describing parties in the ECR as on the opposite end of the spectrum to its Irish republican tradition.
In a statement, the party's chairman Brendan Smith and chief whip Sean O Fearghail attacked Mr Crowley's alliance with Mr Cameron's organisation as unacceptable.
"We are disappointed with Brian's personal decision to join the ECR and by doing so align himself to policies and political parties that directly contradict core principles of the Fianna Fail organisation," they said.
"These include the group's eurosceptic stance and its position on membership of the Eurozone.
"Indeed, a number of the parties in this right wing grouping stand in direct opposition to Fianna Fail's republican ethos and tradition."
The ECR was created in 2009 after UK prime minister David Cameron moved to pull Tory MEPs out of the European People's Party (EPP), the largest and main centre right group in Brussels.
Fianna Fail has been affiliated to the Alliance of Liberals and Democrats for Europe (ALDE) in Brussels since 2009.
Despite voting with them over the past five years, Mr Crowley said he was leaving the liberal grouping to protect his staff and help him better serve his constituents in Ireland.
"The ECR group is now the third biggest group in the EU and its priorities include creating long-term jobs and prosperity for member countries, which matches my policy priority," he said.
He described the group as non-federalist and pro-EU group with a focus on giving a stronger say to national governments rather than EU institutions.
It is understood he was in dispute with ALDE over retaining two Irish staff in Brussels.
Despite the defection, Fianna Fail said it remains a committed member of the liberal group.
They are to decide if any action is to be taken against Mr Crowley after a parliamentary party meeting in Dublin.