Lawyers for Paddy Jackson have indicated he intends to take a libel action against Labour Party senator Aodhan O Riordain.
The former minister tweeted about the Belfast trial shortly after the jury cleared Mr Jackson of rape.
His comment questioning the outcome, which was reposted hundreds of times, made reference to "smug, well-connected middle-class boys".
In a statement yesterday, KRW LAW said it had no option but to issue a notice of intention to sue Mr O Riordain "for defamatory comments".
"I can confirm we have issued pre-action libel correspondence against a named Senator in the Republic of Ireland," said senior associate Marie Hans of KRW LAW solicitors.
"The legal action relates to a tweet sent to a number of other persons before it was eventually taken down.
"We will not hesitate to repeat similar legal action against anyone who, deliberately or otherwise, sees fit to attack our client.
"We are examining carefully every item of social media commentary which seeks to challenge the integrity of the jury's full endorsement of our client's innocence. High Court proceedings will issue shortly in both Belfast and Dublin."
Mr Jackson (26), from Oakleigh Park, Belfast, and his Ireland team-mate Stuart Olding (25), of Ardenlee Street in the city, had denied raping the same woman at a house in south Belfast on June 28, 2016.
Mr Jackson denied a further charge of sexual assault.
Blane McIlroy (26), of Royal Lodge Road, Belfast, had denied exposure, while Rory Harrison (25), of Manse Road, denied perverting the course of justice and withholding information.
On Wednesday, all four men were found not guilty.
Mr O Riordan discussed the tweet with his party leader shortly after removing it from the internet on Wednesday.
Brendan Howlin said the Senator "instantly regretted" posting the message and took it down "immediately".
"He put his hands up, made a mistake, it should haven't have happened," Mr Howlin said.
The hashtag #suemepaddy trended on Twitter yesterday after news of the planned legal action against Mr O Riordain.
Posters used it to support the Senator and the complainant in the case.