Former Irish President Mary McAleese has said she would be heartbroken if Brexit meant she would have to show an identity card when crossing the border.
She fears checks between NI and the Republic of Ireland are an inevitable consequence of the UK leaving the EU.
In an interview with RTÉ following an event at the Edinburgh Festival, she described Brexit as like "pulling a tooth with 10,000 roots".
The Belfast-born barrister was president between 1997 and 2011.
Northern Ireland is the only part of the UK that will share a land border with an EU state post-Brexit.
Mrs McAleese said while she was reassured that the UK does not want a so-called hard border, she was concerned the open border cannot be maintained if there is no customs union and strong immigration controls.
"So far I haven't heard anything apart from soothing words that it will be ok," she said.
"Part of me says that has to be underpinned with something infinitely more concrete than the desire that everything will be alright. We need to start hearing how it will be alright."
Describing the open border as a hard-fought "joy", she said some form of ID would be needed eventually and that would make her "deeply, deeply unhappy".
She added that the lack of a power-sharing executive in Northern Ireland could be "very costly" in the long run, as Brexit was going to "reshape the future for generations to come" and the people and politicians needed to be "feeding the debate".