The Under-Secretary for Ireland (Permanent Under-Secretary to the Lord Lieutenant of Ireland) was the permanent head (or most senior civil servant) of the British administration in Ireland prior to the establishment of the Irish Free State in 1922.
The Under-Secretary's residence was at Ashtown Lodge in Phoenix Park, also known as the Under Secretary's Lodge.
The Dublin Castle administration was the central executive government of Ireland under English, and later British, rule from the Middle Ages until the creation of the Irish Free State in 1922. The name comes from Dublin Castle, which served as the administrative and ceremonial heart of British authority in Ireland. "Dublin Castle" is used metonymically to describe British rule in Ireland, particularly the executive and civil service that governed through the Lord Lieutenant of Ireland and senior officials such as the Chief Secretary for Ireland and Under-Secretary for Ireland.
The administration operated alongside the Parliament of Ireland (which existed until 1801) and the Privy Council of Ireland, implementing policies and managing patronage, taxation, and the civil service. Over time, the Chief Secretary for Ireland became the key political manager, while the Lord Lieutenant largely served a ceremonial and representative role.
View the full Wikipedia page for Dublin Castle administration