The Prime Minister's first visit to the Irish border since the Brexit referendum is too little too late, Sinn Fein has said.
Party vice-president Michelle O'Neill said Mrs May would learn about the "catastrophic implications" of Brexit and hear the "fear and trepidation" of locals living and working near what is to become the UK's only land border with the European Union.
The border remains a crucial sticking point in Brexit negotiations with the EU, amid a stand-off between the UK and Brussels on how to maintain free flow of movement across the 310-mile frontier between Northern Ireland and the Irish Republic.
Mrs May was meeting business representatives on the Northern Ireland side of the border on Thursday afternoon.
Ahead of the visit, Mrs O'Neill branded it "too little, too late".
"She is coming two years after the referendum, she is coming two years after negotiating with her own party," she said.
"I am quite clear what she will hear today, she'll hear about the catastrophic implications of Brexit, the fear and trepidation of the business community in terms of what comes next for them.
"We can't withstand being outside the customs union and the single market.
"Theresa May needs to realise that we will not be collateral damage her for own reckless Tory agenda."
Mrs May is visiting Fermanagh following an invite from Democratic Unionist leader Arlene Foster.
Mrs Foster, whose 10 MPs prop up the Prime Minister's minority government at Westminster, said Mrs May would hear of the challenges and opportunities presented by Brexit.
"This visit will enable Mrs May to speak with people who live, work and travel across the much talked about Irish border on a daily basis," said Mrs Foster.
"She will hear first-hand examples of how people see both challenges and opportunities for their sectors as we leave the European Union.
"For our part, we want to see a sensible exit from the European Union which works for Northern Ireland, the United Kingdom and our nearest neighbours in the Republic of Ireland.
"That must mean that our national Parliament takes back control of our laws, borders and money and that there are no new internal barriers created inside the United Kingdom."
Ahead of her arrival, Mrs May said: "I look forward to hearing views from businesses on the border in Northern Ireland on our departure from the European Union.
"I fully recognise how their livelihoods, families and friends rely on the ability to move freely across the border to trade, live and work on a daily basis.
"That's why we have ruled out any kind of hard border. Daily journeys will continue to be seamless and there will be no checks or infrastructure at the border to get in the way of this.
"I've also been clear we will not accept the imposition of any border down the Irish Sea and we will preserve the integrity of the UK's internal market and Northern Ireland's place within it."
On Friday, Mrs May will deliver a speech in Belfast focusing on how her vision of Brexit, outlined in last week's Government white paper, will impact Northern Ireland and the border.
Mrs May will also hold talks with the region's political parties on the two-day trip, with separate bilateral meetings scheduled across both days.
Northern Ireland has been without a properly functioning devolved government for 18 months due to a bitter fallout between the two biggest parties - Sinn Fein and the Conservative Party's confidence and supply partners at Westminster, the Democratic Unionists.
The Prime Minister will meet a DUP delegation, including Mrs Foster, on Thursday, and will also hold talks with the SDLP.
Party deputy leader Nichola Mallon said Mrs May had to respect the rights of the majority (56%) who voted Remain in Northern Ireland.
Ms Mallon said a free flowing border could only be maintained if Northern Ireland remained in the single market and customs union - options Mrs May has ruled out.
"The SDLP's message to the British Prime Minister will be as direct as it is clear - fully respect the rights of the majority of the people in the North who voted to remain and ensure that we remain in the customs union and single market, and stand up to and reject the reckless attempts by the hard Brexiteers of the Tory party and the DUP to undermine and destroy the Good Friday Agreement," said Ms Mallon.
In further comments ahead of her visit, Mrs May added: "From the start of the negotiations, the UK Government has put Northern Ireland's unique circumstances at the heart of our negotiations. And nothing will undermine our commitment to protecting the Belfast (Good Friday) Agreement.
"I also look forward to meeting political parties on working together to restore stable and effective devolved government for the benefit of everyone in Northern Ireland."