Dungloe Castle was built by the Mac Sweeney Family at end of the fourteenth century on a peninsula of rock jutting out into the Atlantic, in the townland of Cloughglass near Burtonport. Although the site to-day is on an island, it is presumed that when the castle was built, the island was connected to the mainland, and that it was the passage of time that eroded them apart. The castle consisted of a single tower, three or four stories high with walls up to five feet thick and constructed from local granite. This type of dwelling seems to have been popular with Irish chieftains at the time, as they are to be found all over Ireland. Today only the remains of the front wall are to be found, with the rest presumably fallen into the sea or carried away by local landowners for the construction of houses and outhouses.

A member of the Mac Sweeney Family was in residence in the Dungloe Castle during the time of the Spanish Armada in 1588. This Spanish fleet was known as the "Invincible Armada", because the Spaniards at the time thought that it could not be defeated. Their decimation by the English Fleet was therefore unexpected and a great blow to Spain's prestige, the World's most powerful country at the time. The wrecks of these Armada ships dot the coast line, and the remains of unnamed Armada ships lies at a place called "the Culltraigh", only three hundred yards distance from the Mac Sweeney stronghold, Owen Mac Sweeney was reputed to have helped sailors from two Armada ships, to return home to Spain, in defiance of the normal code of practice, of turning them over to the British. The occupants of the castle, like most of the dwellings at the time, made great use of the sea fauna and flora in their everyday lives. The castle became a focal point for trading and bartering, with the result that in 1685, a fair was established near the castle, at a place called "Ashraigh" This enabled the local tenants, and the occupants from the neighbouring islands to come together on a given day, to pay their rent and sell their goods and services. This fair was very successful and was know as the "Dun na Cloiche Glaise" fair, which later became Dun Cloiche fair and was eventually anglicised to Dungloe fair. In 1789, this fair was transferred to An Clochan Liath and it is from this that Dungloe received its present name.

Owen MacSuibhne lost control of both the castle and his titles around the time of the plantation of in 1609 and was replaced by landlords and agents of the Crown. The most notable of these were the Conynaghams, a family that had come to Ireland around 1650. After helping Cromwell defeat the Irish at Drogheda, and by helping William at the Boyne, they were given as a reward, lands in Clare, Meath and Donegal. In Donegal, they were given all the land between Mountcharles and Carrickfin. The Conynahams main residence was Slane Castle in Co Meath, where they stayed for most of the year, making only short trips to Donegal to collect rent etc. On these trips here, they stayed in the Mac Sweeney castle in Cloughglass, where their agents lived the year round.

The Conynaghams used Dungloe Castle as their main residence in Donegal for many years, following their setting up of a new estate. The importance of setting up a seaport and fishing centre for the future viability of the area was evident to them, and thus they turned their attention to the island of Inismacadum, (now known as Rutland Island). This island situated halfway between Arranmore Island and Burtonport, provided a safe and deep anchorage, and now became the focal point of the Conynaghams attention. The resulting neglect of Dungloe Castle led to it falling into disarray, and then a crumbling ruin.

Tourists and locals alike now visit the beautiful beaches, swim in the clear blue water and view the magnificent scenery, close to Dun na Cloiche Glaise. They are unaware that they are standing on a spot, which in days one by, was a hive of activity and a vibrant focal point in the history of the Rosses.