McGregor and Poirier last fought one another in 2014 at featherweight, but the pair stepped on the scales in Abu Dhabi as lightweights this time around.
McGregor, 32, hit the scales at 155lbs exactly – championship weight – while Poirier, also 32, made the lightweight limit of 156lbs.
The card’s co-main event is also a lightweight bout, with the division’s sixth-ranked fighter Dan Hooker welcoming former three-time Bellator lightweight champion Michael Chandler to the UFC.
New Zealander Hooker and American Chandler both weighed in at 156lbs.
All fighters set to compete at the Etihad Arena on Yas Island – nicknamed ‘Fight Island’ – also faced off with their respective opponents.
“Everyone, I’m so excited to be back,” said McGregor after being handed a bottle of Poirier’s signature hot sauce as the cordial build-up between the old rivals continued.
“You’re in for a treat tomorrow, alright? Let’s go.”
Poirier, meanwhile, said: “My whole career has aligned me for tomorrow night, and I’m here to be victorious and get my hand raised by any means necessary.
“I’m leaving it all out there. Let’s go.”
Two of the card’s scheduled bouts were scrapped, however.
Matt Frevola was due to take on Ottman Azaitar, but the lightweight contest was pulled from the card with Azaitar having been cut from the UFC on Friday for giving his and his team’s wristbands to people outside the promotion’s bio-secure bubble, president Dana White said.
Also at 155lbs, Arman Tsarukyan missed weight and his opponent Nasrat Haqparast did not show.
Frevola successfully weighed in as a back-up fighter, however, and will be glad he did; he is now paired with Tsarukyan, who will forfeit 20 per cent of his purse for hitting the scales at 157lbs – although the new bout will take place at that weight.
Elsewhere, women’s flyweight Jessica Eye successfully made weight, having failed to do so ahead of both of her last two fights.
Khalil Rountree, meanwhile, made weight for his light heavyweight bout against Marci Prachnio at the second attempt.
Full weigh-in results
All weights in lbs
Dustin Poirier (156) vs Conor McGregor (155)
Dan Hooker (156) vs Michael Chandler (156)
Jessica Eye (126) vs Joanne Calderwood (126)
Marina Rodriguez (116) vs Amanda Ribas (116)
Makhmud Muradov (186) vs Andrew Sanchez (186)
Matt Frevola (155.5) vs Arman Tsarukyan (157) – 157lbs catchweight bout
Brad Tavares (185.5) vs Antonio Carlos Junior (185)
Sara McMann (135) vs Julianna Pena (136)
Khalil Rountree (206)** vs Marcin Prachnio (205)
Nik Lentz (150) vs Movsar Evloev (150) – 150lbs catchweight bout
Amir Albazi (126) vs Zhalgas Zhumagulov (126)