Former Taoiseach Bertie Ahern says Arlene Foster was "eaten alive" for trying to sell the Brexit deal to her party.
He says the Northern Ireland Protocol is "poisonous" in unionist areas, but the DUP now also faces the possibility of Sinn Féin becoming Northern Ireland's largest party.
Ms Foster will step down as DUP leader and the Northern Ireland First Minister following mounting pressure from MLAs and MPs.
Bertie Ahern says there is a possibility Sinn Féin could occupy the First Minister office following the next Assembly election, which is due to be held in May 2022.
On The Pat Kenny Show, Mr Ahern said the Alliance Party could be the biggest beneficiary from internal rifts within existing parties and the DUP's swerve towards the hard-right.
He said: "There’s a new family in the North coming together who don’t see themselves as either hardline republicans or union loyalists… that group is tending to look to the Alliance and a new way forward.
“I think it will continue to grow, but will it grow enough to keep Northern Ireland stable?"
The former Taoiseach said it's not clear if it's in the DUP's short or long-term interest to win back more support from more hardline, conservative, religious and unionist voters.
He suggested unionist parties are not happy about the prospect of Sinn Féin overtaking the DUP as the largest party.
If that does happen, Mr Ahern said it could potentially create a "wobbly road ahead" for the institutions created under the Good Friday Agreement.
He noted: “There’s always the risk in the North… where people walk from the institutions.
"My own feeling is it would be very difficult if the institutions come down again to get them up.”
Mr Ahern said it took a ‘really good’ Northern Ireland Secretary in Julian Smith to get the Executive up and running again last time after a lengthy break.
"I feel sorry for her"
The former Taoiseach said the swiftness of Ms Foster's departure is surprising, but the recent discontent within the DUP has been no secret.
He observed: “I think all the things coming together, but it was the heat on the ground at constituency level within the DUP.
“She came in in January 2016, and Brexit then happened. She’s had a very difficult period.
“Personally, I feel sorry for her. I got on well with her. I think maybe personally she was a far nicer person than it came across in public.”
Unhappiness among unionist communities with the post-Brexit Northern Ireland Protocol is considered one of the key factors behind the moves against Ms Foster.
Mr Ahern said it's clear unionists made a mistake in supporting Brexit and opposing Theresa May's proposed Brexit deal.
He said: “They were wrong, no doubt about it, and they were sold a pup by Boris Johnson… the difficulty is will they acknowledge that?”
He suggested the support with the DUP seems to be for Edwin Poots to become the next party leader, with Jeffrey Donaldson taking a key role in Westminster.