Bertie Ahern has warned a UK exit from Europe could force a new referendum in Ireland.
The former Taoiseach said the Republic should be as helpful as possible to Britain as it seeks concessions from the EU ahead of a poll on whether to exit as early as next year.
But he recalled his experience of European referenda showed they opened up new issues not envisaged by treaty negotiators and could be exploited by foreign interests.
Mr Ahern said: “Treaty change is difficult, if you have treaty change it means every one of the 27 (other member states) have to ratify in national parliaments or in referendums.
“I don’t see how you can make the changes the Prime Minister (David Cameron) is looking for without some sort of treaty changes.”
The former Fianna Fáil leader served as Taoiseach between 1997 and 2008. In 2004, he held office as president of the European Council during which European leaders agreed a new European constitution.
The British government wants that constitution to be significantly reformed if it is not to leave the EU, touching on issues of sovereignty, competitiveness and immigration.
Mr Ahern supported the Irish Government’s position that Ireland should be a helpful good neighbour to Britain.
But he recalled battles over Irish referenda on the Lisbon and Nice European reform treaties in the first decade of the 21st century.
“When you negotiate these huge agreements and bring it to the people you get different issues,” he said.
“Pollsters said it was about Irish neutrality or about abortion and there was interference by several countries who could not get their own way in their own country so came over to try to influence us in our country.”
Ireland voted twice on the Lisbon Treaty, which centred on the creation of a European constitution, approving it in 2009.
Mr Ahern was in office during the poll on the Nice Treaty, which reformed the institutional structure of the EU to accommodate eastern expansion. The referendum was passed with a yes vote in 2002.
He told a conference in Newry, Co Down, his views had changed since the referendum on Ireland entering the EU.
“I don’t love Europe as much as I did in 1972. The alternative is something that we sure don’t want.”
He said Ireland had strong reasons for wanting its major trade partner the UK to remain within the EU, particularly preserving trade links.
But he acknowledged: “I understand many of the UK’s objectives, the Irish Government have said they are open to considering sensible proposals.”
Those include immigration and benefits.
“Preventing any abuses of freedom of movement will serve to strengthen the freedom of movement in the long run.”
He said there was a “simplistic” view about the impact on trade agreements if the UK pulled out, leaving the Government to negotiate individual agreements with many countries or blocs.
“When it comes down to it (Europe) is still a huge part of our economic life, it is a huge part of trade and business.
“The reason we are successful, the reason we managed to book our work through another recession, the reason we can give good employment and educate people and keep things going North/South is because we are part of a trading block that is the EU.”