Unionism could be an energetic and energising voice in the debate around Irish unity, Sinn Fein president Mary Lou McDonald has said.
Her comments came after a DUP MLA sent a letter warning unionists they may not be able to vote to keep Northern Ireland in the UK.
North Down MLA Alex Easton contacted his constituents asking them to register in case there is a vote on Irish unity.
The letter emerged hours after DUP leader Arlene Foster claimed there was no possibility of a border poll.
Mrs McDonald said people should not “feel anxious” about a border poll.
Speaking outside Leinster House in Dublin, she said: “People shouldn’t feel a sense of threat around the prospect of a border poll or the debate that will lead into it or the Ireland that will emerge from it.
“This will be a collaborative and collective process. We won’t write the blueprint for a new united Ireland.
“Unionism needs to be accommodated and welcomed and become a very energetic and energising voice, and perhaps a challenging voice, in the debate around Irish unity and in the debate around the securing a referendum on that matter.”
Mr Easton’s undated letter includes a electoral form for constituents to get registered.
He wrote: “If for instance, there was ever a referendum called on a border poll on a United Ireland, and you are not on the electoral register, you would NOT be able to vote to save our UNION with the rest of the UK!”