KRW Law claim the Senator made "defamatory comments"
Senator Aodhán Ó Ríordáin has broken his silence over pending legal action from lawyers representing Paddy Jackson.
Mr Jackson, one of the defendants in the recent Belfast rape trial, is set to take legal action against the Seanad member.
Mr Jackson was acquitted of all charges against him.
Senator Ó Ríordáin posted commentary on his Twitter account following the verdict last week.
In a tweet on Sunday, he said: "The matter is now in the hands of my solicitors. I won't be making any further comment."
The matter is now in the hands of my solicitors. I won't be making any further comment.— Aodhán Ó Ríordáin (@AodhanORiordain) April 1, 2018
A statement on Friday from lawyers representing Mr Jackson said they were to issue notice of intention to sue Senator Aodhán Ó Ríordáin for "defamatory comments made by him in the immediate aftermath of the jury's verdict."
KRW Law said Mr Jackon "now finds himself resorting to the Civil Courts in order to seek protective action".
Senior associate of KRW Law solicitors, Marie Hans, added: "I can confirm we have issued pre-action libel correspondence against a named Senator in the Republic of Ireland.
"The legal action related to a tweet sent to a number of others persons before it was eventually taken down."
She said High Court proceedings will issue shortly in both Belfast and Dublin.