St. Mary's Cathedral was designed by the renowned English architect Augustus Welby Pugin, who is said to have gained inspiration from the ruins of Ardfert Cathedral "which is particularly evident in the slender triple lancets in the east and west walls".

Separately the spire and nave were completed in 1907 by the Irish architects Ashlin and Coleman of Dublin, who had designed Cobh Cathedral.

[2] The width of the nave was based on the medieval models to be found throughout Ireland and England.

The west end is very Irish in character, with three tall lancet windows and a very low entrance door beneath.

Many of the original interior features were removed or damaged and this renovation is regarded by some as controversial.