Ms. Smith joined the Irish Defense Forces as a young infantry recruit and transferred five years later to the air corps, where she worked for another five years as a flight attendant on the official government jet, assisting government ministers including Bertie Ahern, then the prime minister.

She converted to Islam in 2011 while still serving in the Irish Defense Forces. She resigned that year, shortly after giving a newspaper interview about her decision to convert, saying she had become disillusioned with a lifestyle of drink, drugs and partying.

She is believed to have traveled to Syria around 2015, despite being on a police watch list as a potential Islamist radical. She married a British jihadi and conceived their daughter . The father is believed to have been killed in the fighting in Syria. Her daughter is now in the care of Ms. Smith’s relatives in her native town, Dundalk.

After she traveled to Syria, she reappeared this year with her infant daughter at a Kurdish-controlled displacement camp for wives and children of Islamic State fighters. Interviewed there by a BBC journalist, she said that she wanted to return to Ireland and denied having used her weapons training on behalf of ISIS or having been involved in any acts of violence or terrorism.

“If you asked me am I going to hurt anyone? No,” she said. “Have I any intentions to do anything? No. I’m just interested in trying to bring my daughter up and get her educated. I don’t even think I’m radicalized. All I know is I just came to an Islamic State and I failed.”

Under Irish law, citizens can, in principle, be prosecuted for terror offenses committed abroad, but strong evidence would have to be produced linking the defendant to specific crimes. In the BBC interview, she denied a report that she had trained girls to become ISIS fighters, saying she never picked up a gun while in Syria.

“Even if I wanted to go fighting, I tried to go fighting, they wouldn’t let me,” she said.

The Irish police said in a statement said that Ms. Smith had been arrested under a catchall provision that is generally used to hold and question people suspected of internal subversion or terrorist activity.