An agreement a year ago between the UK and the EU, known as the Windsor Framework, eased customs checks and other hurdles but didn't go far enough for the DUP, which continued its boycott.
The UK government this week agreed to new changes that would eliminate routine checks and paperwork for most goods entering Northern Ireland, although some checks will remain for illegal goods or disease prevention.
The new changes included legislation "affirming Northern Ireland's constitutional status" as part of the UK and giving local politicians "democratic oversight" of any future EU laws that might apply to Northern Ireland.
The UK government also agreed to give Northern Ireland more than £3 billion for its battered public services once the Belfast government is back up and running.
“I believe that my party has delivered what many said we couldn't,” DUP leader Jeffrey Donaldson said outside the assembly chamber in Stormont.
“We have brought about change that many said was not possible, and I believe that today is a good day for Northern Ireland, a day when once again our place in the United Kingdom and its internal market is respected and protected in our law and restored for all our people to enjoy the benefits of our membership of the union.”