The Latin phrase quoted in the title of this paper was inscribed on the trowel presented to Archbishop Daniel Murray at the foundation stone ceremony for the new church of Our Lady of Mount Carmel in 1825. The phrase ‘resplendent in darkness, shining as the very day’ described the presence of the archbishop, but also the new building which was yet to be realised. This paper explores the architectural characteristics of the ‘grand chapel’ on Whitefriar Street as it developed throughout the nineteenth century, paying attention to the significance of the formal style as well as its relationship to the surrounding built environment. In doing so, it will also explore how this building was seen and understood in its contemporary environment – as a beacon of a new era of Catholic liberty, as a statement of Catholic respectability, and as a lightning rod for anti-Catholic rhetoric and hostility. Building on the work of Brendan Grimes, this paper will examine the significance of classical church design in Ireland in the first half of the nineteenth century.