Mary Louise McDonald is an Irish politician who has served as Leader of the Opposition in Ireland since June 2020 and president of Sinn Féin since February 2018. She has been a Teachta Dála (TD) for the Dublin Central constituency since 2011. She previously served as vice president of Sinn Féin from 2009 to 2018 and a Member of the European Parliament (MEP) for the Dublin constituency from 2004 to 2009.

The Leader of the Opposition in Ireland is a de facto term sometimes used to describe the politician who leads the largest party in the Parliamentary Opposition in the lower house of the Irish Parliament, Dáil Éireann. In the Dáil, the Leader of the Opposition sits on the right-hand side of the Ceann Comhairle and directly opposite the Taoiseach. The role is not an official one and is not recognised in the Irish constitution, nor in legislation.