Aine Davis, 39, was arrested at London's Luton Airport last August after being deported to Britain from Turkey, where he had been convicted of membership of the Islamic State jihadist group.
He appeared at London's Old Bailey on Monday by videolink from prison and pleaded guilty to two terrorism fundraising charges and possession of a firearm for terrorist purposes.
Judge Mark Lucraft said he would sentence Davis on Nov. 13.
Davis was previously linked to the Islamic State cell assigned to guard foreign prisoners in Syria, dubbed The Beatles by hostages because they were known as English speakers.
The group is alleged to have detained and in some cases killed Western hostages in Syria, including U.S. journalists James Foley and Steven Sotloff and aid worker Kayla Mueller.
Davis has always denied being part of the cell and his lawyer Mark Summers said at a preliminary hearing last year that U.S. prosecutors did not charge Davis due to evidence there were only three members of the cell.
Two members, Alexanda Kotey and El Shafee Elsheikh, were been sentenced to life in prison in the United States. A third member of the group, Mohammed Emwazi, died in a U.S.-British missile strike in Syria in 2015.
(Reporting by Sam Tobin; editing by Mark Heinrich)
