He has been a Teachta Dála (TD) for the Dublin Fingal East constituency since the 2024 general election.

He previously served as Minister for Housing, Local Government and Heritage from 2020 to 2025 and a Senator for the Labour Panel from 2011 to 2016.

His brother Eoghan O'Brien is a Fingal County Councillor, representing the Howth-Malahide local electoral area.

[2] Before entering politics, he worked in the Pensions sector with Friends First Assurance Company.

O'Brien was appointed to the Fianna Fáil frontbench as Spokesperson for Sport in January 2011.

[5] He was nominated by the Taoiseach Brian Cowen to the 23rd Seanad on 4 March 2011 to fill a vacancy.

He returned to the Dáil at the 2016 general election, winning a seat in the new constituency of Dublin Fingal.