Birr Castle, Birr, Co. Offaly

Walk up to that crenellated entrance front and you are standing where a Norman fortification once stood guard over Ely O'Carroll, the O'Carroll territory that gave this part of Offaly its name; the building has simply been reclothed, generation after generation, until the working parts of a defensive tower house became the centrepiece of a demesne residence. Few Irish country houses wear their layered history quite so openly on the façade, even if you need to know where to look.

The castle began as a Norman stronghold at the start of the thirteenth century and passed through O'Carroll hands from the fourteenth century until the close of the sixteenth, when it was sold to the Ormond Butlers. It was Laurence Parsons who brought the estate into the family that holds it still, acquiring the land in 1619 and 1620 along with its fortifications, among them a square tower known as the Black Castle standing near the present building. Parsons set to work almost immediately, and his reconstruction of the gatehouse became the core around which the later house grew. The most dramatic transformation came two centuries on, after the Second Earl of Rosse, another Laurence Parsons, opposed the Act of Union of 1800 and, on losing that fight, retreated to Birr and turned his energies to remodelling the house instead, engaging the little-known local architect John Johnston to build a new castellated front facing the demesne to the north rather than the town to the south. The façade seen today was substantially complete by around 1840, its ashlar limestone walls, hoodmoulded windows and crenellated parapet the product of that Gothic Revival campaign, while inside an open well staircase dating from between 1660 and 1681 rises through three storeys beneath a plaster Gothic ceiling, and a saloon built for music between 1805 and 1812 carries a vaulted Georgian Gothic ceiling on columns. The dining room's curving tracery ceiling belongs to a further remodelling of 1836 and 1837, a reminder that this was a house never quite finished, each generation of Parsons layering its own Gothic fantasy over the fortified core beneath.

The Tudor arched porch at the centre of the entrance front, reached up a flight of limestone steps, gives a fair sense of the whole building's method: a recessed doorway with carved stone surround and battened timber doors, topped by a fanlight worked in stone tracery, all of it dressed in the castellated language of the nineteenth century but sitting directly over the footprint of Parsons' seventeenth-century gatehouse. Look too at the variety of window treatments along the façade, from pointed arches with cut stone hoodmouldings to square-headed openings with mullions and transoms, each a small clue to which century's builders were at work on that stretch of wall.