It was never a hospital in the modern sense. The name reflects an older usage: a place of hospitality and care, in this case for old soldiers who had served in the wars and needed somewhere to see out their days. The building was designed to accommodate 300 such pensioners across its three floors, making it also the largest secular building of its period in the country.

William Robinson, who held the position of Surveyor General, completed the main structure in 1684, with the steeple tower following in 1701. The layout follows a courtyard and piazza plan, a formal arrangement borrowed from Continental Europe, with open arcading running along three sides of the ground floor and partway along a fourth. The north front is built of calp stone, a dark Dublin limestone, dressed with limestone, granite, and plaster. At the centre of each range, doorways are topped with tympana, the triangular or curved decorative panels set into a frame above a door, here carved from wood rather than the more usual stone. Above the main entrance are the arms of the First Duke of Ormonde. The building has been compared to Les Invalides in Paris, which Louis XIV had constructed between 1670 and 1676 for much the same purpose, though Kilmainham followed a somewhat more restrained architectural programme. Archaeological work carried out in 1976 ahead of restoration uncovered the remains of cobbled paths in the grounds, along with late eighteenth-century pottery and clay pipes.

The building sits within grounds that are worth approaching on foot from the Liffey side, where the scale of the north front becomes properly apparent. The OPW has been responsible for its restoration, and the building now functions as the Irish Museum of Modern Art, which means the interior, including the Great Hall and Chapel in the north range, is generally accessible during museum opening hours. The arcaded walkways reward a slow circuit; the carved wooden tympana above the doorways are easy to overlook but repay close attention.