Lisa Smith has been arrested on suspicion of terrorist offences following her arrival back in Ireland this morning.
Ms Smith and her two-year-old daughter were on board a Turkish Airlines commercial flight that landed at Dublin airport shortly after 10am.
The 38-year-old former Irish Defence Forces left more than three years ago to join the so-called Islamic State group in Syria.
She's being held at a garda station on the southside under the powers of the Offences Against the State Act.
“On her arrival in Dublin, Lisa Smith was met by An Garda Síochána," Justice Minister Charlie Flanagan said in a statement.
"In the case of Ms. Smith’s child, established procedures for her care will be followed. An Garda Síochána and the Director of Public Prosecutions are responsible for criminal investigations based on facts and evidence in all cases and it would not be appropriate for me to comment on those matters. This is a sensitive case and I want to reassure people that all relevant State agencies are closely involved."
Freelance Journalist Norma Costello, who has interviewed Ms Smith, believes it will come as a surrprise to her.
"We've been given kind of indications that this might happen, but now it's been confirmed," she said.
"She will be detained -- we're not sure for how long. I think Lisa will actually be quite surprised by this turnaround, because when I interviewed her twice in Syria Lisa had no idea that there was any chance she could get arrested for this."
Speaking yesterday, Taoiseach Leo Varadkar said Ms Smith's daughter will be protected on arrival in Ireland.
He added: "In relation to Lisa Smith, the gardaí are going to want to speak to her.
"They may be in a position to charge her and if they do, then prosecution may follow."