Marc Ó Cathasaigh (Irish pronunciation: [ˈmˠaɾˠk oː ˈkahəsˠiː]; born 3 January 1977) is a former Irish Green Party politician who served as a Teachta Dála (TD) for the Waterford constituency from 2020 to 2024.

[4] He lives in Tramore, County Waterford, with his wife Róisín and their three sons.

[5] He was a primary school teacher at Glór na Mara in Tramore and the chair of the Waterford Cycling Campaign.

[6] He was a member of Waterford City and County Council for the Tramore local electoral area from 2019 to 2020.

Laura Swift was co-opted to his seat on the council following his election to the Dáil.