Lisa Smith, 38, once accompanied an ex-president and leader of Ireland on foreign trips as a member of the Defence Forces but went to the war-torn Middle Eastern country in 2015 after converting to Islam and becoming radicalised.
Ms Smith had been living with her two-year-old daughter in a Syrian refugee camp; she is being cared for by her family in Ireland while her mother is questioned at a Dublin police station.
She was deported from Turkey and put on the first scheduled Turkish Airlines flight to Ireland, which landed at the Irish capital's airport shortly before 10.30am on Sunday.
Irish police said: "Today, Sunday 1st December 2019, at Dublin Airport, An Garda Siochana has arrested an Irish Citizen (38 year old female) on suspicion of terrorist offences following her deportation from Turkey.
"She is currently being detained at a South Dublin Garda station under the provisions of Section 30 of the Offences against the State Act, 1939 as amended.
"A child, also an Irish citizen, was in the company of the female and is now being cared for by relatives."
Ms Smith has denied being involved in violence.
Irish broadcaster RTE said she was accompanied by three consular officials from the Department of Foreign Affairs, members of the Army Ranger Wing, and a Turkish security officer on the flight.
She was not in custody for the four-and-a-half-hour journey but Irish police were waiting at Dublin airport for the plane's arrival.
Images of her arrival showed her covered in a pink blanket as she was taken from the aircraft.
She was transported in a dark-coloured van to Kevin Street police station in Dublin.
Her daughter was born in Syria but is an Irish citizen.
Ms Smith is originally from Dundalk in Co Louth, close to the Irish border with Northern Ireland.
She has said the father of her child was a suspected member of IS who died last year.
Ms Smith held a relatively lowly role in the Defence Forces but worked on the official Irish Government jet.
She accompanied former president Mary Robinson and then Taoiseach Bertie Ahern on journeys.
Significant numbers of Europeans left for Syria to fight for and against IS during a bloody war which destroyed the Middle Eastern country and produced millions of refugees.
Irish Taoiseach Leo Varadkar said Ms Smith should have the right of return to Ireland and that removing her citizenship would not be right or compassionate.
Justice Minister Charlie Flanagan said: "This is a sensitive case and I want to reassure people that all relevant State agencies are closely involved.
"A multi-agency network is in place here comprising agency personnel who engage on an ongoing basis with international colleagues regarding emerging practice in relation to the complex issue of radicalisation.
"This network will co-ordinate engagement on a case by case basis as and when appropriate."
In Britain, captured British Islamic State fighters will be brought back to the UK to be put on trial if it is the best place for them to face justice, the national security adviser has said.
Shamima Begum, the Bethnal Green schoolgirl who fled to Syria to join IS in 2015, was stripped of her British citizenship by then-home secretary Sajid Javid, prompting her to take legal action against the UK.
Ms Begum claims she married Dutch convert Yago Riedijk 10 days after arriving in IS territory.
She told The Times that she left Raqqa in January 2017 with her husband but her children, a one-year-old girl and a three-month-old boy, had both since died.
Her third child died shortly after he was born.