Taoiseach Enda Kenny has promised David Cameron that he will be as "supportive and constructive" as possible in his bid to renegotiate the terms of Britain's membership of the European Union.
Following talks in Downing Street, Mr Kenny said that it was in the interests of Ireland and of Europe that Britain remained a "leading player" in the EU.
He said that he wanted to see Mr Cameron's promised in/out referendum once the renegotiation is complete result in a "Yes" vote for the country to stay in.
"We want that to be a referendum that can be carried. It is critically important that Britain stays a central and leading player in the European Union of the future," he said.
"In that we will be as supportive and as constructive as we can. It doesn't mean we will follow you blindly on every issue.
"But insofar as the process is concerned I want to see that leading to a decision by the British people to stay in the European Union because that is where the future for everybody lies."
Mr Kenny's backing was warmly welcomed by Mr Cameron, who has had a distinctly mixed response in his attempts to build support for his renegotiation plan among fellow EU leaders.
The British prime minister has been touring European capitals to explain his thinking ahead of next week's Brussels summit when it is expected to be on the formal agenda for the first time.
But with other EU leaders largely preoccupied with the Greek debt crisis, he acknowledged that there may be limited scope for progress at next week's meeting.
"I wouldn't say that there is going to be any one particular date that is crucial but obviously the forthcoming European Council is an opportunity to get this process under way," he said.
Mr Cameron and Mr Kenny talked for about an hour-and-a-half, enjoying a working lunch of baked ricotta and grilled asparagus, sea bass and vegetables followed by lemon panna cotta with strawberries.
The Prime Minister's official spokeswoman said Mr Cameron "talked the Taoiseach through the areas where he wanted to address the UK's concerns" on Europe.
"The Taoiseach welcomed the PM's approach, the fact that he is seeking to engage with everyone personally, and said that he wanted Ireland to be as supportive as it could be," the spokeswoman said.
"He talked about the fact that quite a lot of what the UK was putting on the table would benefit the whole of the EU, particularly on the issues around competitiveness and reform."
The two leaders also discussed the Mediterranean crisis and agreed there should be more work to "deal with the problem at source" and examine what the EU could do to bring about a stable government in Libya.