Éamon Ó Cuív has confirmed that he resigned as deputy leader of Fianna Fáil because he differed with party leader Micheál Martin over the party's position on the EU fiscal treaty.
Mr Ó Cuív also resigned as spokesperson on communications, energy and natural resources.
"He thought my views on (the treaty) were not compatible with being deputy leader and I have accepted this, because (my views) are different from Micheál Martin's," said Mr Ó Cuív.
"He is the party leader and I do accept that he has a majority for his view in the parliamentary party.
"To me, this whole issue is about the fundamental future of Ireland and I believe it's much bigger than any one individual.
"Therefore it would be impossible for me to stay as deputy leader and not have my heart and soul in what I would consider to be an absolutely fundamental decision that we have to make."
Mr Martin said: "Éamon's refusal to support the Parliamentary Party's position on the Fiscal Compact meant that it was no longer feasible for him to occupy these roles.
"As the party prepares for a momentous Ard Fheis this weekend, it is my hope that Éamon will continue to work for Fianna Fáil's renewal as he has done over the last twelve months.
"Our Party has led Ireland's positive and constructive engagement with Europe for generations and I am determined that it continues in this proud tradition."
Deputy Ó Cuív has said he will attend the Fianna Fáil Ard Fheis this weekend.