The 37-year-old lock pleaded guilty to two charges of assault by penetration and one of sexual assault when he appeared before a court in Cardiff.
The offences against three women took place between October 31 and November 2, two days before the Barbarians-Wales match at the Principality Stadium on November 4.
Ratuniyarawa, who joined French club Bayonne on a short-term deal after London Irish went into receivership and was called up to Fiji's World Cup squad in October, had been due to be on the bench for the match.
His lawyer, Ruth Smith, said her client, a married father of four young children, had been hoping an appearance with the invitational side would lead to a new contract.
The offending "arose out of primarily the consumption of alcohol", she added.
Ratuniyarawa entered not guilty pleas to two further charges of sexual assault relating to one of the three women, which were accepted by the prosecution.
A judge at Cardiff Crown released the player on conditional bail and placed him on the sex offenders register until sentencing on January 9.
