In a letter signed by the woman who had filed the complaint, she offered her "unreserved apology" to Mr Costigan.
"In late January 2019, I made a number of posts to social media sites about Jason Costigan," the letter stated.
"Those posts suggested that Mr Costigan was of poor character, that he had posed a threat to my family, and that he was not fit to be a Member of Parliament.
"I now accept that the posts I made were entirely unjustified. I regret making them, and I regret the hurt that they must have caused to Mr Costigan and his family.
"For those reasons, I offer my unreserved apology to Mr Costigan."