White hasn't decided whether UFC 257 will be held in Las Vegas or in the UFC's coronavirus-secure bubble in Abu Dhabi.
His last meeting with Poirier in 2014 ended in victory for McGregor inside the first round. The official signing by both parties puts an end to negotiations that included potentially shifting the contest to headline UFC 256 on December 12.
Poirier signed his contract last week.
However many withing fighting circles took it with a grain of salt, with McGregor linked to a possible cross-combat fight with Filipino boxing icon Manny Pacquiao before the Poirier bout was confirmed.
McGregor is 3-2 overall since leaving the featherweight division. McGregor relinquished both titles before attempting to defend them.
Conor McGregor continues to be one of the biggest draws in the UFC, even if he hasn't been as active as people would like him to be. McGregor captured the division's undisputed title in 2016 by knocking out Eddie Alvarez, while Poirier captured the division's interim title with a 2019 decision win over Max Holloway.
"I am very grateful to be about to return and get to do what I love again", McGregor, a former two-weight UFC champion, was quoted as saying. His attempt to unify it was ultimately unsuccessful when he too ran into Nurmagomedov, and he too lost by submission. Betting lines now see McGregor as a -220 favorite over his former foe Poirier (+180).
The fight will headline UFC 257 with both fighters signing off on the agreement.