The New York Times reported on Tuesday that McGregor, 30, was accused of sexually assaulting a woman in December in Dublin, and was arrested and released after questioning the next month. He has not been formally charged with a crime; the investigation is ongoing.
UFC officials did not immediately respond to requests for comment.
The report comes less than 24 hours after McGregor announced his sudden retirement from mixed martial arts on social media.
Citing multiple sources, The Times reported that the woman alleged an attack happened at a hotel in South Dublin, west of the Crumlin neighborhood where McGregor grew up. Irish media have reported on the case for months but are barred by law from naming a suspect in sexual assault cases before a conviction.
According to the report, Irish police are not saying whether McGregor is the suspect of the investigation at this time.
"This story has been circulating for some time, and it is unclear why it is being reported now," Karen Kessler, a spokesperson for McGregor's legal team, told ESPN.
"The assumption that the Conor retirement announcement today is related to this rumor is absolutely false. Should Conor fight in the future, it must be in an environment where fighters are respected for their value, their skill, their hard work and their dedication to the sport," Kessler said.
McGregor (21-4) last fought for the UFC in October. He suffered a submission loss to Khabib Nurmagomedov in a lightweight title fight at UFC 229. Earlier this month, McGregor was arrested in Miami on charges of robbery and criminal mischief after he took the phone of someone trying to take his picture, according to a police report.
Before announcing his retirement on Monday, McGregor told "The Tonight Show Starring Jimmy Fallon" he was in the process of negotiating a potential return to the Octagon in July.
Ariel Helwani contributed to this report.