Theresa May will meet Leo Varadkar to update him on her ongoing efforts to get changes to the Brexit withdrawal deal.
The prime minister and taoiseach (Irish prime minister) will have dinner in Dublin on Friday night.
The EU has said it will hold more talks with the UK to help the prime minister get a Brexit deal through the Commons
The meeting will take place after Mr Varadkar meets Northern Ireland’s main political parties in Belfast.
Attorney General Geoffrey Cox will also travel to Dublin for talks with his Irish counterpart.
Speaking in Belfast, Mr Varadkar said it was “not a day for negotiations” but that it was an opportunity to “share perspectives”.
Several cabinet ministers have told the BBC a no-deal Brexit could lead to a vote on Irish unification.
But DUP leader Arlene Foster poured cold water on the prospect, saying that the 1998 Good Friday Agreement sets out “criteria for a border poll, and it hasn’t been met – therefore it will not be called”.
What emerged from the PM’s Brussels trip?
On Thursday, Mrs May met EU leaders in Brussels in a bid to secure changes to the Irish border backstop in the Brexit agreement.
European Commission President Jean-Claude Juncker ruled out legally-binding changes to the backstop clause in the 585-page withdrawal document.
But he said the EU would be open to adding words to the non-binding future relations document that goes with the withdrawal agreement.
Other officials, including European Parliament Brexit co-ordinator Guy Verhofstadt, have said the backstop is “non-negotiable”.
What is the Irish government’s view?
On Wednesday, Mr Varadkar held meetings with top EU officials about the backstop and Ireland’s plans for a no-deal outcome.
He said that while he was “open to further discussions” with the UK government about post-Brexit relations, the legally-binding withdrawal agreement remained “the best deal possible”.
The backstop was needed “as a legal guarantee to ensure that there is no return to a hard border on the island of Ireland”, he added.
Speaking in Belfast on Thursday, Mr Varadkar said “time is running out” to agree a deal, but that work needed to continue in order to ensure agreement was reached.
“When it comes to Brexit this is a negotiation that has the UK on one side and EU on the other,” he said.
“Any negotiation can only happen with Ireland and the EU working together.”
Analysis: Diplomacy over dinner in Dublin?
By Jayne McCormack, BBC News NI Political Reporter
Today the focus shifts from Brussels back to Belfast.
Theresa May left her EU meetings with a promise of more talks, but was told there can be no re-negotiation of the withdrawal agreement.
It’s a message that has been reinforced repeatedly in Dublin.
That’s where Mrs May will go tonight for dinner with the taoiseach, although no-one is expecting a diplomatic breakthrough from it.
It will take place just hours after Leo Varadkar meets the main Stormont parties including the DUP and Sinn Féin.
The Irish government insists any talk of Brexit is a matter for the EU, but it’s impossible for the backstop and the deadlock over Brexit not to dominate today’s conversations in Belfast.
Where are we with the backstop?
It is the insurance policy to maintain an open border between Northern Ireland and the Republic of Ireland unless and until another solution is found.
The UK and EU made a commitment to avoid physical barriers or checks on the border, if no UK-EU trade deal is agreed before the Brexit transition period ends.
Many people are concerned that the return of such checks would put the peace process at risk.
But there has been opposition to the backstop from the Democratic Unionist Party (DUP) and Brexiteer MPs, who believe its terms could keep the UK tied to EU rules in the long term.
Last month, MPs backed an amendment in Parliament calling for “alternative arrangements” to replace the backstop.
A group of Conservative MPs has held talks aimed at finding other Brexit options that would avoid a hard border.
On Wednesday, it emerged that they had scrapped a planned visit to Northern Ireland to meet business representatives, however the invite has since been re-issued.
Business leaders in Northern Ireland are due to attend a meeting at Stormont on Friday with the HM Revenue and Customs (HMRC) border delivery group to discuss post-Brexit border arrangements.
The meeting had been cancelled three times.
Why has Leo Varadkar travelled to Belfast?
The Irish government said Mr Varadkar’s talks in Belfast would provide him with an opportunity to discuss the “ongoing political impasse” in Northern Ireland, which has been without a devolved executive and assembly for more than two years.
He said he had travelled north to “hear the perspective of the main parties”.
“We share common objectives to make sure that there is a deal in relation to Brexit, to avoid a hard border, and to maintain frictionless trade.”
Mr Varadkar’s trip comes days after Theresa May met the parties at Stormont to discuss her bid to make changes to the withdrawal agreement.
How have the Northern Ireland parties reacted?
The DUP were the first party to meet Mr Varadkar, with others due to have talks with the taoiseach later on Friday.
DUP leader Arlene Foster said her party had a “wide-ranging” discussion.
Mrs Foster also said some people were engaging in “project fear” with the Brexit negotiations.
The party’s deputy leader, Nigel Dodds, said the backstop “is the problem”, but would not specify which possible alternative to it that his party is fully backing.
Sinn Féin’s vice-president Michelle O’Neill said her party would “hold the taoiseach’s feet to the fire” when it comes to defending the backstop.
She said he had given her an assurance he would remain firm with his stance.
The party also said they have been calling repeatedly for a border poll, and that they had urged the Mr Varadkar to begin planning for one.
The UUP’s Brexit spokesperson Steve Aiken said there needed to be “level-headed conversations” and that the UUP had told the taoiseach how concerned they are by the terms of the Irish border backstop.
The UUP said it is working on a number of alternative proposals it wants the UK and EU to consider.
What happens next?
Brexit Secretary Steve Barclay and EU negotiator Michel Barnier will hold talks in Strasbourg on Monday, as the EU and UK Brexit negotiating teams discuss proposed changes to the deal.
British sources say the talks will include discussion of the legally-binding withdrawal agreement, the BBC’s Brussels reporter Adam Fleming said.
An EU source said the further talks are an opportunity to listen to the UK’s ideas.
Mrs May and Mr Juncker will meet again before the end of February, to review progress.
The prime minister is expected to put the deal to a vote in the Commons towards the end of February.
She said the plan must change if it is to win the support of MPs who urged her to seek “alternative arrangements” to the backstop when rejecting the deal last month.