US dancer James Blackston, who was also convicted of sexually assaulting another woman in the Japanese capital a month before the assault, was sentenced to three years hard labour in 2013.
DCU student Nicola (21) was found strangled in the Keio Plaza Hotel in Tokyo's Shinjuku district on May 24, 2012.
Blackston's friend Richard Hinds was convicted of her murder and sentenced to between five and 10 years in prison.
Blackston was released in 2015 and returned to the US where he styles himself as a dancer and choreographer.
In a post on his Instagram account earlier this week, Blackston who regularly posts religious themed messages on his account, "announced" that he was to be married.
"We have hours before we are married," he wrote beneath a picture of him and his "fiancée."
"I'm enjoying my fiancée for a few more days then she'll officially be my wife and I am excited to learn, love and enjoy life with you."
Blackston's announcement comes eight years after he was sentenced before Tokyo District Court to three years with labour.
In March of 2013, the then 23-year-old was convicted of sexually assaulting a Brazilian woman in April of 2012 and assaulting Nicola's friend in May of 2012 in the Tokyo taxi that also carried his friend Richard Hinds and Nicola.
Judge Masayuki Yamada said that he had done it to fulfill his "desire".
Both women, referred to in court as 'victim A' and 'victim B', said they blacked out after drinking tequila Blackston had given them.
Blackston, a professional dancer who was touring Japan with R&B singer Ai, met victim B at an after-concert party in the city of Kofu, about 100km west of Tokyo.
The following month he and killer Hinds met Victim A and Nicola outside a Tokyo train station and they went to a bar together.
Victims A and B both say that after drinking Blackston's tequila they woke up hours later in his hotel room with no recollection of how they got there. Victim B, a Brazilian, said she "felt dizzy" and sick after taking the drink. "I thought I might die," she said.
During his trial, Blackston pleaded innocence and said both women were telling "lies" and had willingly gone to his hotel.
He lied that Nicola's friend had demanded sex from him in his hotel room.
He told the court: "I don't know why she lies; I don't hate you for it but... why don't you just say what happened?"
But CCTV from inside the Keio Plaza hotel showed him pushing an unconscious victim A to his hotel room in a wheelchair.
The judge also noted that Blackston claimed he had no sexual interest in victim A. But the CCTV camera in the taxi taking them to the hotel recorded him making explicit references to the two Irish women.
The CCTV camera also recorded Blackston assaulting Nicola's friend.
The prosecution said he initially denied that any contact between him and Victim A took place but changed his story when the police showed him the footage. He then admitted contact but lied that it was consensual.
The judge said neither victim A or B consented and were not in a position to as they were not conscious. He took 150 days off Blackston's three-year sentence for time already served on remand.
The prosecution had requested a four-year sentence. But the judge said that Blackston's young age, his lack of a criminal record and the fact that he had employment as a professional dancer stood in his favour.
Blackston's marriage occurred this week as Nicola's killer Richard Hinds remains in lock-up in Fuchu prison for her murder after he was found guilty in March 2013.
The then 19-year-old was given a sentence of a minimum of five years and a maximum of 10 with labour.
The keyboard player has now been imprisoned for more than eight years in total.
Due to poor behaviour while in custody, it is not expected he will be released until the full 10 years expires later next year.
Passing sentence in 2013, the chief judge said Hinds showed "no remorse" and "tainted the honour" of his 21-year-old victim.
The court took account of the killer's age, but the judge said he was entirely to blame.
He said: "The victim was only spending a night out with her friend… after a concert they had looked forward to. She bears none of the fault for the consequences of that night."