Kilkenny Castle was, for over five centuries, the ancestral home of the Butlers. The Butler family are of Anglo-Norman descent, and were founded in 1177 by Theobald FitzWalter, after he was made Chief Butler of Ireland by King Henry II.
Kilkenny Castle itself is situated in the pseudo-city of Kilkenny, in county Kilkenny, in the south-east of Ireland. The town of Kilkenny was originally, like many Irish towns, a monastic settlement, the name Kilkenny meaning "The Church of Canice" (from the Irish Cill Channaigh). The modern city includes Saint Canice's cathedral, which dates from around 1270, and a round tower which may even date back to Canice's 6th century monastery. Kilkenny was granted city status in 1609, by James I, back in the days when being a city was more of a state of mind than a reflection of the population.
The first structure on the site of the castle, a wooden motte and bailey castle, was built there by Strongbow, a.k.a. Richard FitzGilbert de Clare, in 1172, to command the crossing of the river Nore. On his death, his son-in-law, William Marshall, Earl of Pembroke, took it and the surrounding town as his home, and at some stage during the early 13th century, he built the first stone structure on the site, a square castle with four towers, three of which are still standing as part of the modern castle.
The castle remained in the hands of Marshall's family until 1391, when it was purchased by James Butler, 3rd Earl of Ormond. The Butlers were a family of great importance in Ireland's history; they controlled half of the province of Munster (Ormonde translates as "East Munster", from the Irish Oir-Mhumhain); the family held several different titles (Duke, Earl and Marquis of Ormonde, Chief Butler of Ireland); they were cousins of royalty (two of Henry VIII's wives were related to the Butlers, and so both Elizabeth I and James VI were cousins of the family); and also, because of the honour first bestowed on Theobald FitzWalter, they were entitled to 10% of all wine imported into Ireland, a tithe which they received annually. As the Butlers' fortunes rose, so did the profile of their adopted home; the Anglo-Norman Parliament met there from 1293 to 1408, and during the English Civil War (1642-1648), Kilkenny was the capital of the Roman Catholic Confederacy.
The Butlers lived in the castle up to as recently as 1935, when the castle was sold to the city of Kilkenny for the nominal sum of £50. During their residence, the Butlers undertook many changes to the castle, gradually transforming it from a medieval castle to more of a Victorian manor. The largest single change was probably when one of the four sides of the castle was completely demolished, following severe damage sustained in a siege by Oliver Cromwell's army in 1650. This left the castle's structure as it is today, occupying three side of a square, with the fourth facing a beautiful view across parklands and gentle hills. Another major change came in 1820, when William Robertson remodelled the entire castle in a "feudal revival" style for the first Marquis of Ormonde; thankfully, he left the beautiful classical-style gateway, dating from the early 18th centruy, intact. More changes came during the mid-19th century, when the picture gallery was remodelled, and filled with beautiful portraits by the likes of Van Dyke, Kneller and Lely, as well as very ornate painting along the roof beams. A Gothic staircase was also added to the castle during the same period.
As mentioned above, the castle was sold to the City of Kilkenny in 1935; in 1967, the castle and grounds were given over to the state, and are now managed by An Dúchas, the Irish National Heritage Council. The castle has been renovated extensively since then, at great expense to the state, who did all they could to authentically replicate the carpets, wall hangings, furniture, and (slightly ostentatious) golden ceilings that made the castle such a stately home in its hayday. The castle is now open to the public, on guided tours only; the tour takes in the magnificent gallery I mentioned above, which contains portraits of the Butlers of old, some of whom bear a marked resemblance to yours truly in a certain light. It also goes through old bedrooms, one of the towers, and a beautifully restored sitting-room/study. The castle kitchens now house what I'm told is a popular restaurant, and there is also a public gallery in the basement of the castle, which aims to show the best in Irish and international contemporary art.
A great website for tourist information about Kilkenny, as well as history and other background information.
Another good site containing some historical content; this one also covers Carlow, Waterford, Wexford and Tiperary, and has some quite in-depth information.
This article gave me some information on the various architectural and interior-design changes that had been made to the castle.
More little factoids about Kilkenny castle.
This writeup dedicated to the castle-loving wertperch - come over and visit some time! :)