An unidentified woman claims that the 30-year-old athlete assaulted her at the Beacon Hotel in Dublin last winter, the New York Times reported.
Irish media have reported on the alleged assault since last year, although they did not name McGregor. Irish media outlets are not allowed to name individuals connected to rape cases, unless those individuals have been convicted.
Police investigating the alleged assault have obtained closed-circuit camera footage from the hotel, the New York Times reported, citing an individual with information regarding the case. In addition, law enforcement officials have retrieved evidence from the hotel room McGregor stayed in last December, although the nature of that evidence is unknown. In addition, the people who spoke to the Times revealed that McGregor was arrested on January 17, questioned by police and then released without being charged with a crime while the investigation continues.
On Tuesday, McGregor announced his retirement from the UFC on Twitter.
"I wish all my old colleagues well going forward in competition,'' he wrote. "I now join my former partners on this venture, already in retirement."
According to Karen J. Kessler, a New Jersey-based publicist for McGregor, the athlete's retirement is not related to the investigation.
"This story has been circulating for some time, and it is unclear why it is being reported now,'' Kessler's statement to the New York Times reads. "The assumption that the Conor retirement announcement today is related to this rumor is absolutely false."
On March 11, McGregor was arrested in Miami Beach and charged with robbery and criminal mischief after being accused of stealing a cellphone from a fan trying to take a picture of him.
In January, the two-division champion was suspended by the Nevada Athletic Commision from fighting for six months and fined $50,000 over his participation in a post-match brawl after his October 6 bout in Las Vegas against Khabib Nurmagomedov, the reigning UFC Lightweight Champion.
The Russian athlete had emerged victorious in the fourth round of the fight with McGregor. After the fight ended, Nurmagomedov rushed out of the octagon and attacked McGregor's teammate, Dillon Danis, who, according to the fighter, had been insulting him throughout the match. The Irish and Russian fighters' teammates also got involved in the melee, Sputnik reported.
In July last year, McGregor was also charged with disorderly conduct after attacking a bus carrying fellow UFC fighters who were leaving the Barclays Center in Brooklyn, New York, after a press event. The athlete eventually pleaded guilty to the charge.