Lisa Smith, 38, was arrested by Irish police when she arrived at Dublin Airport.
According to Reuters, authorities in Ireland had been trying to repatriate Smith for months, concerned about the well-being of her two-year-old daughter.
Police said the girl is now being cared for by relatives in Ireland.
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Smith is subject to questioning by authorities for up to 72 hours before she is formally charged or released, the Irish Times said, adding detectives from Ireland’s Counter-Terrorism (International) units “have already accumulated a large file” against her.
“They have also gathered information from foreign intelligence services on her movements in the past seven years,” it added.
The 38-year-old allegedly traveled to Syria to join the Islamic State after the group’s emergence in 2014, where she married an Islamic State fighter.
However, she denies taking part in any terrorist activities.
If she is found guilty of membership to the Islamic State, she will receive a maximum sentence of seven years in prison, according to Ireland’s terrorism laws, which were expanded in 2005 to include foreign terrorist groups.
Moreover, if Smith is indicted on charges of engaging in terrorist recruitment or training, she faces up to 10 years behind bars.
Many nations in the European Union fear that due to a lack of evidence of criminal wrongdoing, Islamic State supporters could be quickly released once they appear in court after returning home.