The chapel in Dublin Castle, built between 1807 and 1815, was one of the most impressive ecclesiastical Gothic buildings of the pre-Pugin revival in the British Isles. It was commissioned by the viceregal establishment following the Act of Union between Great Britain and Ireland in 1801, and was closely associated with Church of Ireland objectives for post-Union Protestantism in Ireland. This essay investigates the patrons’ ambitions for the chapel, and discusses its design and execution by Francis Johnston, successor to James Gandon as the foremost architect of public buildings in Ireland. Reviewing the chapel within the context of the Union, the essay argues that the viceregal administration and the Church of Ireland were concerned to assert their authority and define their values, and that these were expressed in Gothic revival architecture which grafted progressive appreciation for medieval models onto Georgian taste, and in a comprehensive and unprecedented scheme of ecclesiastical sculpture. Ireland's political position within the Union was ambiguous, but it is argued here that the rebuilt chapel projected both unionist and imperialist gestures, and that, culturally, it was an expression of Britishness.