Éamon Ó Cuív has announced that he intends to stay in Fianna Fáil, but will make no further comment on the fiscal treaty referendum.
Deputy Ó Cuív had been warned not to campaign against the treaty by the party leadership.
Speaking to journalists at Leinster House, Deputy Ó Cuív repeated his criticism of the treaty, and of what he called "a badly designed monetary union".
However, he said he was faced with a choice to keep his views quiet, or leave the party.
Mr Ó Cuív said it was one of the most difficult decisions of his political career.
He said he saw no party other than Fianna Fáil to represent his views.
The weekend events in France and Greece showed the people of Europe do not agree that there is only one way forward, he added.
Mr Ó Cuív said in February that he would be voting against the treaty and resigned as deputy leader and communications spokesperson.
Mr Ó Cuív received a letter at the weekend from party whip Seán Ó Fearghaíl asking him to refrain from media interviews campaigning for a No vote in the fiscal treaty referendum.
Mr Ó Cuív said the letter had a clear implication that he would have to leave the party if he did not accept its contents.
However, Mr Ó Fearghaíl said the letter did not mention potential expulsion.
Speaking on RTÉ's Six One, Mr Ó Cuív said he believes the treaty is one of the biggest decisions the country is going to make.
In weighing up his decision to stay in Fianna Fáil, he said he looked at the long-term achievements he might make in politics in terms of policies and he believed it is through a party like Fianna Fáil that he will make his contribution.
Mr Ó Cuív revealed that, other than a few texts, there had been no communication with the party leadership since he received a letter from them last Friday as he reached his decision.
Mr Ó Cuív said he would never have questioned his position with Fianna Fáil in the past, but in recent times he was very concerned about being silenced on the issue of the treaty.
He said he was particularly swayed by large numbers of Fianna Fáil supporters and members who were like-minded with him, and wanted him to stay.
He said differences in opinion were the life and blood of political parties and he would continue to voice his opinions on issues within the party.
He will await instructions from his party leadership on whether he needs to remove some material relating to the treaty from his website, he added.
Fianna Fáil Chief Whip Seán Ó Fearghaíl has welcomed the Galway West TD's decision.
A statement from Mr Ó Fearghaíl said: "I would like to acknowledge and welcome my party colleague Éamon Ó Cuív's confirmation that he will remain a member of Fianna Fáil and will not be campaigning against the party in the referendum campaign.
"We welcome this clarity and Éamon's determination to play a constructive role in the renewal of the party."