The Department of the Taoiseach (Irish: Roinn an Taoisigh) is the government department of the Taoiseach (prime minister) of Ireland. It is based in Government Buildings, the headquarters of the Government of Ireland, on Merrion Street in Dublin. The Department was created in 1937 and replaced the Department of the President of the Executive Council, the department of the Irish head of government under the previous prime ministerial title in the Irish Free State. Both departments are descended from the Chief Secretary for Ireland's Office, originally based in Dublin Castle, which was the administrative office of the dominant politician of Cabinet rank in the British government in Ireland in the nineteenth and early twentieth centuries. The head of the Department of the Taoiseach (sometimes also written as the Department of An Taoiseach) is known as the Secretary General of the Department.