The Paisley club moved quickly for Reilly after he became surplus to requirements at Tynecastle, while they have managed to retain Demetriou, who spent the second half of last season with the Buddies after signing from Doxa Katokopias in the Cypriot first division.
Reilly moved to Hearts ahead of the club’s return to the Ladbrokes Championship, signing a three-year contract but managed only 32 appearances and five goals, spending last season on loan at Dunfermline in the Championship.
The 24-year-old has previously worked with St Mirren manager Jack Ross, during the duo’s time in Gorgie.
Stelios became somewhat of a cult hero in Paisley despite making only 15 appearances. He played a key role in the club’s survival scoring a crucial double to secure a 2-0 win against champions Hibs.
The London-born Cypriot has played for a variety of clubs in Curpus as well as spells in Bulgaria and Sweden before arriving in Scotland.