British Prime Minister David Cameron has confirmed a referendum on Britain's membership of the European Union will be held on 23 June.
Mr Cameron briefed his ministers at a Cabinet meeting on Saturday morning after leaving Brussels with what he described as a "historic" deal on the UK's future relationship with Europe.
Speaking outside Downing Street, Mr Cameron warned that leaving the EU would be a "leap in the dark" as he urged voters to back his reforms.
Meanwhile, Taoiseach Enda Kenny has allayed fears that Irish people living in Britain could be affected by the curtailing of migrant welfare benefits.
Speaking on the campaign trail from South Mayo, shortly after his return from Brussels, he insisted no Irish workers would suffer from the deal.
Mr Cameron confirmed his cabinet had backed his plan but individual ministers would be free to campaign on either side.
Following his return from a marathon EU negotiating session in Brussels, Mr Cameron said leaving would threaten Britain's "economic and national security".
"Those who want to leave Europe cannot tell you if British businesses would be able to access Europe's free trade single market, or if working people's jobs are safe, or how much prices would rise," he said.
"All they're offering is a risk at a time of uncertainty - a leap in the dark."
Key elements of the deal include agreements over welfare payments to migrants, Britain's right to opt-out of the eurozone, and a promise the UK would have stronger sovereign control over laws made in Brussels.
Labour leader Jeremy Corbyn dismissed the renegotiation as a "missed opportunity" and claimed the changes Mr Cameron has negotiated are "largely irrelevant to the problems most British people face".
Now that a deal has been agreed, ministers are free to declare which side of the debate they will support.
Theresa May has confirmed she will back Mr Cameron and join the In camp, as have other high-profile ministers, such as Mr Osborne, foreign secretary Philip Hammond, defence secretary Michael Fallon and transport secretary Patrick McLoughlin.
However justice secretary Michael Gove has defied the PM and joined the campaign for Britain to leave.
Mr Gove said it "pained" him to disagree with the PM, adding: "But I cannot duck the choice which the Prime Minister has given every one of us.
"In a few months' time we will all have the opportunity to decide whether Britain should stay in the European Union or leave.
"I believe our country would be freer, fairer and better off outside the EU."
Northern Ireland Secretary Theresa Villiers has also declared for the Out campaign.
Fianna Fáil spokesperson on Foreign Affairs and Border Region Development Brendan Smith said, "the fact that the Secretary of State for Northern Ireland would campaign for an exit is deeply worrying.
"Secretary Villiers is entitled to her own views on this matter and that must be fully respected. However I believe it is clear that a Brexit would be bad for Northern Ireland and bad for Irish–British relations," he added.
Democratic Unionist Party Arlene Foster says "in our view we see nothing in this deal that changes our outlook. Therefore we will on balance recommend a vote to leave the EU".
