Mary McAleese says she is 'really worried' about the consequences if Britain votes to leave the European Union.
She says 'a hard border' between the North and the Republic would be inevitable, and is concerned about the effects on the peace process.
The former president has appeared in a campaign video for the 'Irish for Europe' campaign group.
She says it is wishful thinking to assume that the positive aspects to being inside Europe, could continue afterwards.
"There will be different rules applying, the days of the pillars that allow freedom of movement, that organised tariffs and free-trade, that will all be in the mix for re-negotiation.
"I would hope, everybody would hope, that everything would stay the same, that the good things would stay the same - we'd wish that, we can't anticipate it."