The 20-year-old has played just once in PNE's first-team, that back in December 2018 as a substitute against Aston Villa.
He's had a number of loan spells in search of first-team football, including one last year with Waterford in the League of Ireland.
O'Reilly recently returned from playing on loan at Stalybridge Celtic.
St Pat's won the FAI Cup in November and finished as League of Ireland Premier Division runners-up last season.
Their new season starts in March, with pre-season just starting for them.
O'Reilly's loan spell will run until the end of July which will allow him to play in St Pat's Europa League qualifiers - finishing runners-up in the Premier Division booked them their slot.
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