This castle takes its name from a rath now known as Round Hill, lis meaning “fort,” and mor “great.” It is situated on the right bank of the River Blackwater, about four and a half miles north-by-east of Tallow, in the county of Waterford.

When Henry II. visited Lismore in 1171 he seems to have formed the plan of turning the ancient and famous Abbey of Mochuda into a fortified episcopal residence; hence in 1179 Milo de Cogan and Robert FitzStephen were sent by his directions to choose the site for this stronghold, which was to act as a protection against the “mere Irish.”

In all probability the tapering tower, now known as “Sir Walter Raleigh’s Tower,” formed part of the ecclesiastical buildings. It is constructed of rude rubble, and has plain loops and cornices. The entrance is on the second floor, and this leads into buildings of later date, so that it has no external doorway. A somewhat similar tower was destroyed by fire prior to 1864, which may (with the one now standing) have protected a gate between the upper and lower courts.

The outer wall, with its beehive-roofed bastions at the corners, and the old gate, which has its archway decorated with chevrons, are likely to have been of twelfth-century construction.

The entrance to the castle is by “the Riding House,” so called from its having formerly been guarded by two mounted sentries, the niches for the horses being still shown. This leads into a long shaded avenue, flanked by high walls which extends to the opening of the lower courtyard. Over the gate are the arms of the first Earl of Cork, and the motto “God’s Providence is our inheritance.” “King John’s Tower” is situated to the right of the entrance, and the “Carlisle Tower” on the left. This latter is about 240 feet in height, and was erected to commemorate the Lord Lieutenancy of the Earl of Carlisle. It is constructed of coarse rubble. The stone for the dressing of its windows and for other parts of the castle was quarried at Chatsworth, and brought over in specially chartered vessels.

The “Flag Tower” flanks the north-east angle, and the oldest wing faces east towards the garden. The upper court is reached through a passage on the west of the entrance, and here Sir Walter Raleigh’s tower is situated to the north.

The whole fortress is built upon a rock, which on one side descends precipitously to the Blackwater, the base being clothed with trees.

The Earl of Cork employed “a free Mazon of Bristol” during his alterations. The modern improvements were principally designed by Sir Joseph Paxton.

The main door has an Ionic porch executed in Bath-stone, said to have been the work of Inigo Jones.

The hall is square, and is used as a billiard-room. A stone stairway ascends on the left side of the entrance. The present banqueting hall was originally a chapel, and has a Gothic roof of open woodwork. The drawing-room contains a large bay window overhanging the river, known as “King James’s Window.” During his stay in Ireland in 1689, he spent a night in the castle, and dining in this room, he approached the window, but started back when he saw the depth below.

The sword and mace of Youghal are exhibited in the hall, where is also the Pastoral Staff of Lismore, which was discovered built up in a doorway of the castle with a valuable Irish manuscript book, since called the “Book of Lismore.”

In 1181 Cullen O’Cullane, and O’Phelan, Prince of the Decies, attacked the fortress, which had been somewhat hastily constructed, and they killed fifty to eighty of the garrison and razed the stronghold.

Prince John, Earl of Morton, landing at Waterford in 1185, rebuilt the castle on a larger scale.

Nine years later the men of the Decies took the fortress by surprise and killed Robert Barry, brother of Giraldus Cambrensis. In the autumn of the same year (1189) the Irish, finding they could not hold the castle, decided to destroy it, but they afterwards surrendered it upon terms. From this time it appears to have been an episcopal residence for some four hundred years.

In 1218 the Bishop of Waterford wrote to Henry III. complaining that the castle of Lismore had been taken from him by Thomas FitzAnthony and Griffin FitzGriffin. The King ordered that it should be restored to the bishop.

When Robert de Bedford was elected Bishop of Lismore the fortress was transferred to him, but not without the Bishop of Waterford declaring it belonged to his see. Bishop de Bedford appealed to Rome, and after a dispute of twelve years it was finally confirmed to the see of Lismore.

In 1271 Lord Justice Audley came on a visit to the castle, and Roger de Mortimer, after he landed at Youghal as Lord Justice, was the guest of Bishop Fleming at Lismore in 1317.

Some time before his resignation in 1589, Meler Magrath, Bishop of Lismore and Archbishop of Cashel, granted the castle to Sir Walter Raleigh at a rent of £13 6s. 8d., and three years later Sir Walter sold it to Sir Richard Boyle, afterwards the first Earl of Cork, who restored and enlarged it.

His great son, the philosopher, was born in the castle in 1626.

The stronghold was besieged three times during the civil wars of 1641. It was first attacked by five thousand Irish troops under the command of Sir Richard Beling, and was successfully defended by Lord Broghill, the Earl’s third son.

The following year an unsuccessful attempt was made to burn it by the Irish.

In 1643 it was again besieged by Lieutenant-Colonel Purcell with seven thousand foot and nine hundred horse. This time Captain Hugh Croker commanded the garrison. The Earl records in his diary that the rebels demanded the surrender of the fortress, but “we retorned them defyance.” None of the defenders were killed, but their enemies lost about three hundred in killed and wounded. The following month cannon was brought to bear on the stronghold, and a breach was effected in the brewhouse, but it was quickly repaired with earth, and the fire from the castle was so great that the enemy did not dare to storm the opening. The guns were then shifted to the south-west, and the orchard was attacked, but the shots from the turrets protected the curtain wall.

After a siege of eight days, the Earl’s sons, Lords Dungarvan and Broghill, landed at Youghal and made a treaty with Lord Muskerry for a six days’ truce. Of the besiegers twenty were killed, while the defenders escaped injury.

The great Earl died in 1644. The following year the castle was again besieged, this time by troops under Lord Castlehaven. Major Bower, with a garrison of a hundred of the Earl’s tenants, managed to kill five hundred of the besiegers and to make terms before they surrendered.

The 4th Earl of Cork died without male heirs in 1753, and Lismore Castle passed to his eldest daughter, Lady Charlotte Boyle, who had married the 4th Duke of Devonshire in 1748. It thus passed to its present owner, the 8th Duke of Devonshire, who entertained King Edward VII. and Oueen Alexandra at the castle in 1904.