O'Reilly was a farmer and an activist in the Nationalist Party and was elected to Kilkeel Rural District Council.
He stood unsuccessfully for the Irish Anti-Partition League in the 1948 Armagh by-election.
[1] O'Reilly was elected in the 1958 Northern Ireland general election, representing Mourne, holding the seat until the abolition of the Parliament of Northern Ireland in 1972.
[2] He became prominent in the 1960s, when he tried to have Orange Order parades through Kilkeel rerouted.
[1] O'Reilly stood unsuccessfully in South Down at the 1973 Northern Ireland Assembly election.