An Irish citizen aligned to Islamic State deported from Turkey along with her two-year-old daughter was arrested on her return to Dublin on Sunday on suspicion of terrorist offences, Irish police said.
Ms Smith has continually denied training young women to fight for ISIS and says she never killed anyone.
Pictures show Ms Smith covered in a pink blanket as she was taken from the plane.
Justice Minister Charlie Flanagan said: "This is a sensitive case and I want to reassure people that all relevant State agencies are closely involved".
Turkish Airlines flight 1975 from Istanbul landed shortly after 10am this morning.
Her return was confirmed from a variety of sources, including those also onboard the commercial flight Smith and her daughter had seats booked on.
She has said the father of her child was a suspected member of IS who died a year ago.
Ireland's child and family agency as well as "different partners" are now looking into the "welfare of the child", he added.
She's being held at a garda station on the southside under the powers of the Offences Against the State Act.
She'll now be arrested and questioned about her decision to go to Syria to join the Islamic State group in 2015.
She accompanied former president Mary Robinson and then Taoiseach Bertie Ahern on journeys.
When asked whether she was prepared to be prosecuted when she returned to Ireland, Smith said: "Well I know they'd strip me of my passport and stuff and I wouldn't travel and I'd be watched kinda - but prison?"
In Britain, captured British Islamic State fighters will be brought back to the United Kingdom to be put on trial if it is the best place for them to face justice, the national security adviser has said. It is believed she is now being detained close to Syria's border with Turkey.
We are actually obliged under global law to repatriate her back to the Republic of Ireland so we didn't actually have a choice in that.