McGregor won the first meeting by first-round knockout in September 2014. Poirier leveled the score with a second-round knockout in January.
Both fighters then agreed they'd have to embark on a tiebreaker bout.
"1 a piece now with a Trilogy bout for all the marbles! Wow! Exciting! Not a trilogy I was expecting, nor the tactical affair I was anticipating, but I'd be lying if I said this wasn't meant to be," McGregor wrote on Feb. 3 via Instagram. "This is exactly how this was always meant to be!"
Poirier replied (h/t TMZ.com): "You know we are going to do it a third time."
The third matchup will once again be crucial in the lightweight division, which has been waiting for a new top dog to emerge with Khabib Nurmagomedov having retired and not committed to a full-time return despite plenty of speculation in recent months. The Russian remains atop the 155-pound division.
It's hard to predict how the latest meeting will play out.
McGregor scored a dominant first-round knockout in the first bout, but that was seven years ago. Poirier owned the second clash, but it came with his opponent coming off a yearlong layoff and entering the Octagon for just the third time since late 2016.
A lot will likely depend on whether The Notorious is better prepared to defender Poirier's leg kicks, which were the major story at UFC 257.
"The kicks accumulated, for sure," McGregor told reporters. "I knew when I saw he had Thiago Alves in his corner."
If he's more prepared to limit those effective shots to the lower body, the question then becomes whether his American counterpart will have another trick up his sleeve for the trilogy fight.
It sets the stage for an intriguing encounter that could be a turning point for both fighters in their efforts to climb atop the lightweight division.