She was said to have been treated at Bordeaux University Hospital and to have been interviewed by Irish police officers assisting the local force as part of the World Cup security operation.
A rape allegation was then filed on Monday morning at the Bordeaux central police station.
The local prosecutor’s office told Le Parisien an investigation had been opened into the alleged incident, with police said to be seeking three Caucasian men, one of whom was described as “very athletic”.
A statement added: “The investigations are underway and in particular the genetic examinations on the victim and the clothing, the collection of testimonies and the study of the video protection of the neighbourhood.”
In a statement, Irish police said officers “currently deployed in France for the duration of the Rugby World Cup tournament to assist Irish people visiting France are liaising with local authorities in relation to an alleged incident”.
Foreign Office travel advice for British nationals travelling to France for the World Cup makes no mention of a heightened risk there of sexual assault.
Its general safety and security advice for France focuses on the threat of terrorism, warning those involved were “very likely to try to carry out attacks” there.
It also cites “several cases of serious assault” on the Paris Metro, including the line serving the Stade de France.
A statement from the Irish Rugby Football Union read: “The IRFU is aware of the disturbing news emanating from Bordeaux regarding an incident involving an Irish supporter at the weekend. We are shocked and extend our deepest sympathies to the victim. The IRFU wants all fans to have a positive experience supporting the team in France and we urge all travellers to take care and look out for each other.”