[1][2] O'Reilly describes herself as being "born a republican socialist" because of her families' involvement in trade unionism.
Her father Mick O’Reilly was a trade union official as well, and was a member of the Connolly Youth Movement, the youth wing of the Communist Party of Ireland, before joining the Labour Party, to which the union he worked for was affiliated.
[3][4] O'Reilly herself worked as an organiser with the SIPTU trade union for a decade before her entrance into politics.
[5][6] O'Reilly was selected as the Sinn Féin candidate for Dublin Fingal in March 2015 in preparation for the 2016 Irish general election.
[8][9] At the general election in February 2020, O'Reilly was re-elected in Dublin Fingal, topping the poll with nearly 25% of the first-preference votes.