Nendrum – An ancient Monastery with the oldest tide-mill in the world

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Nendrum Church & Path
Nendrum Church & Path

The Monastery of Nendrum

The monastery of Nendrum or Inis-Mochaoi was founded in the fifth century by St Mochaoi.

It is said that, as St Patrick was travelling from Saul to the parish of Bright near Downpatrick, he met a youth herding pigs. The boy was called Caolan. The two conversed and Caolan became a disciple of the saint. In due course he was consecrated a bishop.

Caolan was much admired and revered by the local people and was known affectionately as Mochaoi, meaning ‘my dear Caolan’. After Patrick converted the pagan chieftain MacCuill, Mochaoi established a church and monastery on this little island in County Down. Mochaoi’s death is recorded as 23rd June 497.

Location of Nendrum on the map
Location of Nendrum on the map

Viking Attack

The last reference to Nendrum church in the Annals is 974AD. It is generally believed that the monastic site was attacked and destroyed by the Danes.

It was these invaders who actually named the lough by the name we know it today. Strangford comes from Strang Fjord meaning ‘fjord of the strong currents’. The original Gaelic name was Lough Cuan – ‘the quiet lough’.

Viking longships were regular visitors to Strangford Lough during the ninth and tenth centuries with coastal and isolated monasteries viewed as prime targets.

Nendrum Round Tower with Strangford Lough
Nendrum Round Tower with Strangford Lough
Ruins on the Lough
Ruins on the Lough

Benedictine Monks

In 1178, John de Courcy, the Anglo-Norman adventurer (see Carrickfergus Castle) after his military success in County Down granted Mahee island to an order of English Benedictines. These monks came from the religious house of St Bee’s, Cumberland, then known as St Bega of Coupland.

“But when victory smiled upon the assailants, the leaders felt it their duty or their interest to make amends, according to the spirit of the age, for their excesses, by munificence to religious institutions. And in this, then fashionable, mode of purchasing off sins and obtaining forgiveness from heaven, John de Courcy distinguished himself beyond many others…this knight devoted a portion of his lately acquired wealth and influence to the erection of new religious communities, as well as the endowment of those already existing; taking care at the same time to maintain a connexion with the sister country, by affiliating these houses to some kindred institution in England”

Rev William Reeves, A Description of Nendrum commonly called Mahee Island, 1845
Nendrum History
Nendrum History

Although the English monks did not seem to remain long, the church was still a place of worship in the twelfth century. It is mentioned in the Papal Taxation records of 1306. However, when a new ecclesiastical building was constructed in the village of Tullynakill in the barony of Castlereagh Lower, the old chapel at Nendrum was abandoned.

Discovery of the Ruins

Nendrum lay undisturbed and undetected until 1844 when Irish antiquarian the Rev William Reeves uncovered the ruins whilst researching history for his book The Ecclesiastical Antiquities of Down, Connor and Dromore published in 1847.

The area was recognised as of being of huge archaeological importance and has been the scene of many excavations ever since.

The main ruins consist of three concentric dry-stone wall enclosures known as cashels. The area contained within the outer wall is about 6 acres.

Outer Cashel
Outer Cashel
Inner Cashel & Tower
Inner Cashel & Tower

The Round Tower

In the central cashel are the remains of a round tower. The remnants are 4.25m tall but it is reckoned it would have reached a height of about 27m and been clearly visible in the surrounding countryside. It was constructed in the tenth century.

Round towers are free standing, cylindrical stone buildings used as bell towers, for storage and as places of refuge in troubled times. They taper gently inwards and were topped with a cone-shaped roof. The only door, usually facing east, would be high above the ground. This would be accessed by a rope ladder, which would be quickly pulled up when the last person was safely inside.

Within the tower would be 5-7 storeys with wooden floors reached by a ladder. The windows were narrow slits to lessen the danger from arrows or fiery missiles. The top floor under the roof often had 4 windows at the cardinal points, this room contained the bells. These would be rung to call the faithful to prayer and as warnings if the enemy were seen on the horizon.

Nendrum Tower
Nendrum Tower

The Stone Church

The stone church was built around the same time, probably on the location of an earlier wooden construction. Although quite small it would have accommodated 100 monks. In the thirteenth century, when in use as a parish church, the chancel was added.

At the south-east gable of the church is a canonical sundial dated to 900AD. This divides the day into a fixed timetable for prayer and services, rather than marking off hours. This was reconstructed from fragments found in situ during H C Lawlor’s excavations in 1924.

Nendrum Church ruins
Nendrum Church ruins
Nendrum Church illustration
Nendrum Church illustration
The Sundial & Tower
The Sundial & Tower

The Nendrum Cross

The flag-stoned path to the church is lined on either side by stone–lined graves. These date from the seventh and eight centuries and were the final resting place for the monks. Many originally were marked with large slabs inscribed with a cross known as the Nendrum Cross.

Nendrum Cross
Nendrum Cross

The Second Cashel

Within the second cashel are the remains of 7 circular buildings thought to be workshops, including one which may have been a bronze foundry. Knives, pottery, pins and brooches have been found here. A bronze-coated iron bell was also uncovered near the outer wall. These artefacts are now stored in the Ulster Museum.

Also in this area are the remnants of a rectangular building. Within this were discovered inscribed stonework known as ‘trial pieces’, bone carving tools and several slate styluses leading to the idea that this building functioned as a school or scriptorium. Clearly Nendrum was flourishing both as a religious and as an educational centre.

“Nendrum gained a distinguished reputation as a school; it is claimed that Finnian of Movilla and Colman of Dromore were among its pupils”

Ailbhe Mac Shamhrain, Dictionary of Irish Biography

The site also includes signs of gardens, a well, a quay, orchards and pastures to provide the monks the means of growing their own food and maintaining livestock.

Ruins on the Lough
Ruins on the Lough

Discovery of the Worlds Oldest Tide Mill

At the location of an ancient Celtic monastery on Mahee Island the oldest excavated tide mill in the world was discovered in 1999. (A tide mill uses the power of the tide going in and out to turn water wheels which in turn big mill stones to grind wheat or corn into flour). This remarkable find was uncovered by a team from the Ulster University, who were surveying the inter-tidal zones of Strangford Lough.

Using dendrochronology (the analysis of ancient timber) the mill has been dated to 619AD. It depended on the water power stored in a man-made millpond, which refilled twice daily with the tide. The monks built a dam 110m by 60m. This was lined with clay and the seaward boundary reinforced with wood and wattle revetments. The millhouse was situated on the east bank and when the sluice gate was opened the tidal water was forced down a narrow chute which acted like a power-hose. The gushing water hit the spoon-shaped paddles of the horizontal millwheel underneath. This turned the upper and lower millwheels and ground the grain. This is only the most basic of explanations of this amazing feat of archaeological engineering!

Nendrum Tide-Mill
Nendrum Tide-Mill

For more information, please see Harnessing the Tides Thomas McErlean & Norman Crothers 2007

Harnessing the Tides by Thomas McErlean & Norman Crothers
Harnessing the Tides by Thomas McErlean & Norman Crothers

Nendrum Today

Nendrum monastic site as well as providing a wealth of archaeological information is a place of calm and beauty. The discovery of the tide-mill shows us the ingenuity of these holy men and their closeness to nature and the living world around them. They were able to utilise the earth’s resources without destroying them and to live in peace and ‘oneness’ with their surroundings.

“The discovery of the tide mills opens a new chapter in the archaeology of Nendrum begun by Reeves in 1844 when he rescued this important monastery from historical obscurity…. (this) contributes to the respect and admiration felt for the achievements of our ancestors and brings us closer to the reality of their lives. Above all else, it highlights that Nendrum still holds many secrets”

Thomas McErlean and Norman Crothers


The Nendrum monastic site is located on Mahee Island, Strangford Lough, County Down BT23 6EP

A narrow causeway leads onto the island, there is a small carpark and the site is free to visit. There is also a visitor’s centre.

Nendrum Gallery

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