Iran and six world powers have reached a deal on the Iranian nuclear programme after intense negotiations in Geneva.
The deal, while welcomed internationally, has been condemned by Israel.
Iran is to curb its nuclear activities - initially for six months - in return for limited relief from sanctions.
The White House says Iran has committed to halt uranium enrichment above purities of 5%.
US President Barack Obama welcomed the deal saying that the Iranian nuclear programme had been halted for the first time in decades.
Iran agreed to reduce its stockpiles of enriched uranium and open up its nuclear facilities for inspection, he said.
This would mean "substantial limitations which will help prevent Iran from building a nuclear weapon".
US Secretary of State John Kerry said there were no illusions about the risks that Iran might not follow through on its obligations following an interim agreement.
"Now the really hard part begins and that is the effort to get the comprehensive agreement, which will require enormous steps in terms of verification, transparency and accountability," Mr Kerry said.
Iranian Foreign Minister Javad Zarif said the deal was only a first step but an important achievement. He insisted Iran retained its right to enrich uranium.
Tánaiste Eamon Gilmore said it was "an important deal for global peace and security".
A joint statement from European Union foreign policy chief Catherine Ashton and Mr Zarif said the agreement was forged "thanks to a sense of mutual respect and the determination to find a way forward which is beneficial to all of us".
However, the statement underlined that the agreement is just the first step in a longer process.
"The implementation of this first step creates the time and the environment needed for a comprehensive solution which remains the shared goal and on which talks will begin soon."
"We look forward to swift implementation which we will jointly monitor in close coordination with the IAEA.
"Finally, today's agreement is a significant step towards developing our relationship in a more constructive way," the statement added.
Iran: Deal restores dignity
Mr Zarif in separate remarks insisted his country had an "inalienable right" to enrich uranium, a right the US denies exists not just for Iran but for any country.
But Mr Zarif said the agreement was an important first step to putting an end to a "rather sad chapter".
"I believe it is important that we all of us see the opportunity to end an unnecessary crisis and open new horizons based on respect for the rights of Iranian people and removal of any doubts about the exclusively peaceful nature of Iran's nuclear programme.
"These have been and will remain our objective and I am confident that through co-operation we can move forward.
"This is only a first step. It is an important achievement, but this is a first step. We need to work together based on the same principles on which we started, principles of equal footing, mutual respect and common benefit, so that we can put an end to this unnecessary and rather sad chapter."
He added: "I hope that as we go along we can start restoring the lost confidence. The Iranian people demand respect for their rights but demand respect for their dignity."
Israel condemns deal
Israeli Prime Minister Benjamin Netanyahu's government denounced the world powers' nuclear agreement with Iran as a "bad deal" to which Israel would not be bound.
"What was achieved last night in Geneva is not a historic agreement, it was a historic mistake," Mr Netanyahu told his cabinet in public remarks.
"Today the world has become a much more dangerous place because the most dangerous regime in the world took a significant step towards obtaining the world's most dangerous weapon," the Israeli leader said.
Yet Israeli officials stopped short of explicitly threatening military action that could further isolate Israel and imperil its alliance with the US, saying more time was needed to assess the accord.
"This is a bad deal. It grants Iran exactly what it wanted - both a significant easing in sanctions and preservation of the most significant parts of its nuclear programme," an official in Mr Netanyahu's office added.
"The economic pressure on Iran could have brought about a much better deal which would have dismantled Iran's nuclear capabilities."
The west and Israel fear that Iran has been seeking to develop a nuclear weapons capability.
Iran denies this, saying its nuclear programme is a peaceful energy project.
The US says the agreement halted progress on Iran's nuclear programme, including construction of the Arak research reactor, which is of special concern for the west as it can yield potential bomb material.
It would neutralise Iran's stockpile of uranium refined to a fissile concentration of 20%, which is a close step away from the level needed for weapons, and calls for intrusive UN nuclear inspections, a senior US official said.
But the deal still appeared to fall far short of Mr Netanyahu's demand for a total roll-back of the Iranian nuclear programme.
"You stand and shout out until you're blue in the face, and you try to understand why they're not listening. The world wanted an agreement," Finance Minister Yair Lapid, a member of Mr Netanyahu's security cabinet, told Israel's Army Radio.
"We also said that a diplomatic accord would be good. A diplomatic accord is certainly better than war, a diplomatic accord is better than a situation of permanent confrontation, just not this agreement," Mr Lapid added.
He said Israel had to pore over the deal: "For example, we still don't understand exactly what stepping up the monitoring [on Iran's facilities] means. This is a detailed matter. God really is in the small details."
Tánaiste: Important agreement for global peace
The Tánaiste warmly welcomed the deal and commended those who took part in talks for their "patient and diligent efforts which have now yielded such an important agreement for global peace and security".
"However, there should be no doubting the significance of what has now been achieved in Geneva," Mr Gilmore said.
"A road map for resolving diplomatically this vital international issue and preventing the possible further proliferation of nuclear weapons has now been set out and will hopefully result in a comprehensive agreement in the coming months," he added.
Mr Gilmore said that while the deal was only "a first step towards a comprehensive agreement on Iran's nuclear programme" he urged both Iran and the international community "to ensure that this vital agreement is fully and faithfully implemented over the next six months".