Reports emerged over the weekend of St Pat's interest in 20-year-old O'Reilly.
It is understood that the loan deal will run until June and be reviewed then.
O'Reilly's contract at PNE runs until the summer of 2023 so time is still on his side at Deepdale.
He recently returned from a loan spell at Stalybridge Celtic and was reluctant to drop back into non-league for a further loan spell..
They have just signed former PNE striker Eoin Doyle from Bolton Wanderers on a three-year deal.
O'Reilly was on loan in the League of Ireland in the first-half of last year, playing for Waterford.
North End had been hoping to sort a loan deal in League One or League Two in this country but found clubs were reluctant to take younger players with little first-team experience.
Some of PNE's other younger players could go out on loan this month.
Jack Baxter has already extended stay at Radcliffe until the end of the season.
Ethan Walker is back from a spell at AFC Fylde, with Joe Rodwell-Grant and Lewis Coulton having returned from Bamber Bridge.
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