Prime Minister Theresa May has been accused of "switching her language" over the physical border between Northern Ireland and the Republic after Brexit.
In spelling out her plans for the exit from the European Union, Mrs May has said that she wants a "seamless, frictionless border".
She has repeatedly said she does not want a return to the "borders of the past".
Mrs May said that it was an "essential part of daily life" that people have the ability to move freely across the border.
Former Taoiseach Bertie Ahern told the Observer that "any kind of physical border" would be "bad" for the peace process.
He said: "(May) seems to be switching her language. She's saying not that there'll be no border, but that the border won't be as difficult as to create problems.
"I worry far more about what's going to happen with that. It will take away the calming effects [of an open border."
Mr Ahern, who was a key figure in the Northern Ireland peace process, said he had concerns for the peace process.
He said: "Any kind of physical border, in any shape, is bad for the peace process.
"For the nationalist community in Northern Ireland, the Good Friday Agreement was about removing barriers.
"With so many other issues, there is a real concern... the only way (of) doing this will be a hard border."
Mr Ahern raised issues on how to use technology to maintain an open border.
"I haven't found anyone who can tell me what technology can actually manage this," he said.
Mrs May is set to trigger Article 50 next month, which will formally begin the withdrawal negotiation process, as long as her Brexit Bill passes the House of Lords.
In my view, this election will tell us how mature we have become as a democracy because, for the first time since devolution returned 19 years ago, voters have the ultimate democratic choice: if you are happy with the performance of your government, reward those parties by giving them another mandate, or, if you are not, vote in the Opposition and make clear you expect better.
The DUP wants to see pro-Union representation maximised at the forthcoming election. We will work with other parties who receive sufficient support to make them eligible for a place in government. However, it does not require a joint electoral platform with another party to form a coalition government alongside them, or even to work closely and positively with them for the good of all our citizens.