The pair are joining a major nationalist conference being held in Dublin on Saturday to rally support for Irish unity.
Andrew Clarke, 26, and Peter Adair, 21, grew up with religious and cultural influences associated with a British identity.
The pair both attended the Royal Belfast Academical Institution grammar school in the city centre but did not know each other before becoming involved in Ireland's Future. He said it "all just seemed bizarre", adding: "It shocked me hard enough that it made me question everything." "I would say Brexit was definitely the turning point. I definitely would not be in favour of Brexit. For the UK as a whole I don't think it's a good idea but I think it's quite damaging for Northern Ireland," he said.
He said that when the UK was part of the European Union, people's national identity in Northern Ireland could be more "fluid". He said his views have "moved steadily along to different points in the political spectrum" over the years and he would now consider himself an Irish republican.