Dublin Castle was originally built at a defensive fortification during the Norman period. It evolved into a symbol and citadel of British rule for centuries, housing the mechanisms of the British government in Ireland. The Viceroy of Ireland (Lord FitzAlan-Howard) and the representative of the British crown resided in the castle and can be seen in this newsreel getting into a car and exiting the gateway to a jeering crowd.
In the courtyard, soldiers on parade hold their guns raised ready to fire salute (it is unclear if these are British troops). Michael Collins, the Chairman (Prime Minister) of the Provisional Government of Ireland created under the Anglo-Irish Treaty, accepted control of Dublin Castle. The handover symbolised the end of British rule in Ireland, even though the Irish Free State would remain part of the Commonwealth until 1949.