One grand door, three large windows, a compact Neo-Classical facade measuring just over fifteen metres square, and that appears to be everything. It is not. Behind that restrained exterior sit sixteen rooms arranged across three floors, and the windows that seem to light a single grand interior are, in fact, subtly curved panes of glass concealing internal partitions, each one serving several separate rooms at once. Only two panels of that imposing north-facing door actually open. The whole thing is, in the most deliberate and accomplished sense, an architectural fiction.

The building was designed by Scottish architect Sir William Chambers for James Caulfield, the 1st Earl of Charlemont, as an ornamental feature for the gardens of Marino House. Work began in 1759 and was completed around 1775, placing it firmly within the Neo-Classical tradition, a style that drew on ancient Greek and Roman forms to project refinement and learning. The name itself comes from Italian and translates roughly as "the small house by the small sea," a modest title for something so carefully considered. In plan, the Casino takes the form of a Greek cross, with pairs of columns framing each of the four projecting elevations. Inside, the basement holds a kitchen and service rooms; the main floor contains reception rooms decorated with fine plasterwork ceilings and elaborate hardwood parquet floors; above that, a top storey accommodates servants' quarters and a state bedroom. A tunnel once connected the Casino directly to Marino House, though it has since been blocked.

The Casino sits in Marino, on the north side of Dublin city, and is managed as a heritage site with guided tours available. The interior is the real draw here, so a visit that goes no further than the exterior misses most of what makes it genuinely peculiar. It is worth looking closely at the windows before going in, noting how ordinary they seem, and then reconsidering that impression once you are standing inside one of the rooms they serve. The contrast between what the building presents and what it actually contains is not incidental; it was the entire point.