[2] O'Reilly attended college in Marino Institute of Education and is now a primary school teacher in Castleknock, Dublin.
His father Terry, a former garda from Gleann Lára (English: Glenlara), a townland near Belmullet, County Mayo, in the West of Ireland, is club chairman of Seán MacCumhaill's.
[7] O'Reilly started Rory Gallagher's first match in charge of the county, a 2015 Dr McKenna Cup away defeat to Derry.
[17][18][19] He also started the 2015 All-Ireland Senior Football Championship qualifier defeat of Galway at Croke Park.
[30][31] O'Reilly also started the 2016 All-Ireland Senior Football Championship qualifier defeat of Cork at Croke Park and the next game against Dublin at the same venue.
[32][33] O'Reilly started the opening fixture of the 2017 National Football League against Kerry, scoring a point and winning a 24th-minute penalty which was converted by Michael Murphy.
[39] O'Reilly started the 2017 Ulster Senior Football Championship quarter-final victory against Antrim and the semi-final loss to Tyrone, scoring 0–1 in the latter game.
[43] Under the management of Gallagher's successor Declan Bonner, O'Reilly started in the opening fixture of the 2018 National Football League against Kerry in Killarney, as well as the next game against Galway.