The ballroom is a fitting conclusion to a description of Iveagh House. It was the last of the great rooms to be constructed and at 60 feet by 35 feet by 40 feet high is also the largest. It was designed by William Young in 1896 and is typical of his classical style at its most ostentatious. It is a vast chamber public in scale and furnishings, the walls awash with polychromatic panels of Algerian onyx, in Carrara marble and Irish alabaster frames.
The room is built to a tripartite plan, the centre dominated by a dome and the overall effect is rather like some vast and magnificent tepidarium of the ancients. It is said to have cost more than £30, 000 at the time of building. The design of the room mixes many stylistic sources. The marbled walls are inspired by 18th century interiors, perhaps ones at Versailles or at Holkham or Houghton in Norfolk. But much of the decorative detail in the room is in the Italian mannerist style of the late 16th century.
This style is seen particularly in the two great semicircular panels of plasterwork on the walls beneath the dome, where two symmetrical figures, one bearing a lute, the other a tambourine, are set amidst tendrils of foliage and strapwork which squat beneath a central cartouche, a bearded head and feathered wings. The plasterwork in the room was executed by the firm of Darcy and was praised in its time for returning to the technique of stucco modelling in situ rather than relying on pre-cast ornament. However on close inspection it is clear that though the work is indeed hand-finished, moulds dictated the basic forms.
The two-tier chimney-piece facing the windows is notable. It follows the revived Elizabethan fireplace fashion established by the English architect Richard Norman Shaw, but the alabaster chosen for the execution of the design has deficiencies as a material. As on the engaged alabaster columns around the walls, the detailing of the chimney-piece is inexpressive rather like chiselled soap
The Iveagh House ballroom is not only remarkable for its size but also for the completeness of its furnishings. The most splendid feature is the set of window hangings executed by Morrison of Edinburgh - cherry-red damask is overlaid with strapwork outlines in dark red velvet and gold thread. and applique sprays of flowers embroidered in silk. The built-in seat furniture is original too, in French regence style and delightful bays for 'sitting out' from dances are provided at either end of the room.