Castleknock Castle

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Castleknock Castle

County Dublin

Castleknock Castle Nov2018.jpg
Castleknock Castle atop the motte
Grid reference: O08553655
Location: 53°22’6"N, 6°22’11"W
Condition: Ruined

Castleknock Castle is a ruined Norman castle located on the grounds of present-day all boys independent school Castleknock College, in Castleknock, County Dublin.


"The position of the castle is commanding, and its two deep ditches, and the ruins of its massive walls, bespeak its former strength. The Castle itself is thickly clad with ivy, and the entire hill covered with large and spreading trees. The whole is now reserved ground, enclosed with a strong fence. The solemn gloom of the place, its dark winding walks, and the profound silence that reigns around, make it a delightful solitude."[1]

A mound may have preceded the present mediæval Norman structure. The polygonal keep was the notable feature of the castle. Attached to it was a large squat building. A curtain wall, interspersed with towers, surrounded the castle. There is a moat or ditch constructed around the castle. Today, the site is surrounded by trees while the ruins are seldom visible from the road except in winter. The earliest extant of the castle by Francis Place also shows it in ruins, but somewhat less dilapidated than at present.

There is also a small mound to the west of Castleknock College buildings known as Windmill Hill. There is a water tower there now which was built originally as an observatory by a previous owner, Simon Guinn.



There is evidence of the site's importance prior to the erection of the castle in the Norman period. An ancient pagan cromlech was discovered.

"In the year 1861, an ancient Cromlech, or Druid's altar, was discovered in the interior of the old Castle when digging the grave of the Rev. Thomas Plunket.

The workmen, coming on a large flat stone, found it too heavy to remove, and immediately commenced to break it. They succeeded after great difficulty, but on detaching a portion, they found, to their surprise, an empty space beneath, and a human skeleton lying at full length. The head and larger bones were almost perfect, and with them were small heaps of dry, whitish dust. The men not understanding the nature of their discovery, placed the bones a little aside, and continued their work.

It was not till the grave was filled up, and it was too late to remedy the evil, that the whole matter came to light.

From the description given by different persons who were present, there is no doubt that the discovered grave was one of those ancient Cromlechs, or altar tombs, which were used as burial places for kings or notables during the Pagan times.

The skeleton in this case was so old that the admission of air caused a portion of the bones to fall into dust; this accounts for the small heaps of whitish dust which were found with the larger bones."[1]


"In the year 726", say the Four Masters, " died Congalach of Cnucha." In the old translation of the annals of Clonmacnoisc, he is called " Konolagh of Castleknock." In the Annals of Ulster we read " Congalach Cnucho moritur; and in the Annals of Tigernach "Congalach Cnuchaensis moritur." We know nothing respecting Congalach, but that he died at his fort, Cnucha, towards the beginning of the eighth century."[2]

Castle origins

The castle was founded by the Norman knight, Hugh Tyrrel, who was later created Baron of Castleknock. He chose this location near the end of the esker which stretches from Galway to Dublin. Built on two mounds of the esker, it commanded the route into Dublin from the west.

Castleknock was the final rallying point for the forces of the last High King of Ireland, Rory O'Connor. He failed to drive the Cambro-Normans from the area around Dublin in 1171.

At that time the old fort underwent many changes. Tyrrel strengthened his fortress with all the improvements of modern warfare, and in a short time the Norman castle stood aloft in grim defiance, with its heavy battlements and deep double ditch. The battering ram could not approach it, and the missiles thrown against it fell harmless to the ground 'as hailstones from the rounded shield.'[1]

Foundation of abbey

The Abbey of St. Brigid was founded where the Protestant church now stands, by Richard Tyrrell, second Baron of Castleknock, in 1184, and continued to flourish until the suppression of the monasteries, when it was demolished, and a Protestant church built on the site. In ancient times Castleknock furnished two canons to the Cathedral of St Patrick, and still today two prebends of St. Patrick's derive their titles from "Castrum Noc ex parte diaconi, et Ca-strum Noc ex parte praecentoris".

Capture by Robert the Bruce

The Bruces advanced on Dublin (1316). A short time before, Edward Bruce had been proclaimed King of Ireland at Dundalk, and thinking the time had come for the expulsion of the English, he invited his brother Robert to his assistance. The King of Scotland landed in Ireland with a select body of troops, and, being joined by his brother, marched to besiege Dublin with 20,000 men. The first exploit on approaching the city was the taking of Castleknock. It could not be expected that the old fortress, long deemed impregnable, could long hold out against the hero of Bannockburn. Bruce entered, making Hugh Tyrrell prisoner, and fixed there his headquarters.

The rejoicing by the rebels and the Scots at Castleknock was short-lived. Bruce soon perceived that Dublin was fully prepared for a siege, and well provided with provisions from the sea. Moreover, the ardour of the citizens caused him to relinquish all hope. After remaining a few days in the Castle, he released Tyrrell on payment of a ransom, and retired from the city. He halted again at Leixlip as if to resume his campaign, but after a brief pause recommenced his march towards the south, and soon after left Ireland, leaving his brother to continue the war.[1]


A romantic legend has grown up around Castleknock of 'the Lady of the Castle', Eibhleen O'Brinn. The take recounts that in the early 16th century, Hugh Tyrrell (the last of the name) ruled in Castleknock. During his absence, Hugh's wicked brother Roger carried off Eibhleen, the fair daughter of O'Brinn, or O'Byrne, a Wicklow chieftain and confined her in the turret of the castle. At dead of night, the maiden heard steps ascending the stone staircase that led to her apartment, and fearing a fate worse than death she slit her own throat with a breast-pin, and bled to death.

Turlogh O'Brinn was no rebel chieftain but dwelt in peace in the Pale. One of the Knights of St John, who had an establishment in Dublin, marched towards Castleknock to avenge the crime. Tyrrell met his the enemy in open battle before the castle, and was slain.

It was long a popular belief, that, at the hour of midnight a female figure, robed in white, might be seen moving slowly round the castle. This, they said, was Eibhleen, and they called her "The Lady of the Castle".

The War of the Three Kingdoms

In 1642, the Earl of Ormond, commanding 4,000-foot and 500 horse, besieged Castleknock, and found it defended on behalf of the Confederate rebels by Lady de Lacy, aunt of the Earl of Fingal. An account tells that Lady de Lacy found her position hopeless and burned the castle in the midst of a suicidal defence.

Whatever the truth of the account of the siege, the castle was partially dismantled after the War of the Three Kingdoms when considerable artillery damage had been done to the castle.


  • Dónal MacPolin and Peter Sobolewski, Blanchardstown, Castleknock and the Park, 2001, Cottage Publications ISBN 1-900935-22-8
  • James O'Driscoll, Cnucha: A history of Castleknock and district, 1977, privately issued
  • Jim Lacey, A Candle in the Window, 1999 Marino Publications ISBN 978-1-85635-552-0
  • Tadhg O'Keeffe, Mediæval Irish Buildings, pp 230-231 2015 Four Courts Press ISBN 978-1-84682-248-3

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