Irish premier Enda Kenny has urged Britain to remain within the European Union.
The Taoiseach, in London for a series of engagements including talks with prime minister David Cameron, insisted Britain has much to gain from its membership.
"We see the British relationship with the EU as being a two-way relationship - Britain benefits from its membership of the EU, and the EU is better off with Britain as a leading member making a valued contribution," Mr Kenny said.
Ireland is more than half way through its six-month EU presidency term.
Its time at the helm also marks the 40th anniversary of both Ireland and Britain joining the EU.
Mr Kenny said while the country's future is closely tied with Europe, its connection with Britain - its "nearest neighbours" - was stronger than ever.
Speaking during a whistle-stop trip to London, the Taoiseach insisted both nations were like-minded on numerous European issues such as the single market and trade.
He made the comments at a business event at the city's Mansion House, where he also vowed that Ireland would act impartially during negotiations on the controversial financial transactions tax.
He said the state would use its position as EU president to facilitate the discussions, but would strike a careful balance in respecting both sides of the debate.
Ireland and Britain have both opted out of the tax, which is expected to generate up to €35 million (£30.5m) a year from the financial industry.
France and Germany are among 11 states that have signed up to the tax, which will see levies imposed on trades in shares and bonds.
Later, during a lunch at the British-Irish Chamber of Commerce at Savoy Place, Mr Kenny said Ireland looks to the British market as a place where its business could contribute to the state's national recovery.
"For our part, Britain is our biggest market for key sectors such as food and beverages, and of course for tourism," the Taoiseach said.
"Another way of looking at the scale and importance of the British market is that a tiny loss of exports to Britain - between a half and one per cent - would immediately wipe out the hard-won increase in our exports to the developing Brics (Brazil, Russia, India, China and South Africa) markets."
Mr Kenny was also attending the London School of Economics and meeting David Cameron in Downing Street, where they were to review progress on a joint statement they signed last year aimed at developing British-Irish relations.
The pair were also planning to discuss current unrest in Northern Ireland following protests over the flying of the Union flag.
In an address to the London School of Economics later, Mr Kenny said Ireland and Britain had travelled a long distance together within the EU.
"The UK is an enormously important partner for us in Europe," the Taoiseach said.
"We greatly value the contribution you make, and we look forward to it continuing long into the future. It is good for Europe, good for Ireland and, not least, good for the UK."
Following the talks at Number 10, Mr Cameron said the two governments were determined to go "further and faster" in deepening relations.
"We are not letting our foot off the pedals; we want to go further and faster with this relationship and we have tasked our teams to identify more we can do to strengthen collaboration in specific sectors to boost growth," he said.
Both leaders said they had instructed top civil servants to work up policy.
After the meeting, the pair went to the House of Commons for what the prime minister called "a small St Patrick's Day celebration".
The Taoiseach joked that he had handed over "the key" to beating Wales at rugby - ahead of the Six Nations decider with England in Cardiff.
Ireland are the only team to have beaten Wales in this year's tournament so far.
"I'll make sure I pass it on," Mr Cameron quipped back.