The Mansion House's most famous features include the "Round Room", where the First Dáil assembled on 21 January 1919 to proclaim the Irish Declaration of Independence. On 21 January 1969 a special fiftieth anniversary joint session of Dáil Éireann and Seanad Éireann assembled there and was addressed by the President of Ireland, Éamon de Valera.
Externally, the distinctive metal portico over the main door was erected for the visit of Queen Victoria in 1900.
Famous visitors to the mayoral residence include:
In the 1930s and 1940s, plans were made to demolish the building, and all other buildings on the block on which it is located (which covered an area on Dawson Street, Molesworth Street, Kildare Street and the North side of St. Stephen's Green), to enable the building of a new Dublin City Hall. However the decision of the Government to erect a new Department of Industry and Commerce on a site on the block, on Kildare Street, led to the abandonment of the plans.
In August 2006, the loyalist paramilitary Ulster Volunteer Force claimed they had planted a bomb in the Mansion House in 1981, in an attempt to wipe out the Sinn Féin leadership at their party conference of that year. The claim led to a security alert at the house, as police and army searched for the 25-year-old bomb.