A crannóg is an artificial island, usually originally built in lakes, rivers and estuarine waters, and most often used as an island settlement or dwelling place in prehistoric or medieval times. The name itself may refer to a wooden platform erected on shallow floors, but few remains of this sort have been found. The name crannóg, anglicized "crannoge", is from Old Irish "crannóc", from crann, tree.
Crannogs are most common in Ireland, where at least 2000 examples are known, They are also very common in Scotland, with at least 600 sites known. However, it is likely that these are underestimates, and it is very likely that many more undiscovered sites still lie hidden underwater, or in reeds, carr woodland or other wetland environments around lakeshores and edges. Crannogs today typically appear as small, circular islands, between 10-30m in diameter, covered in trees and bushes; isolated from browsing livestock, they are often tree clad. Originally, crannogs may have taken many different forms. However, the classic image of an ancient crannog is of a small island, surrounded or defined at its edges by a post or oak plank palisade and on top of which is a roundhouse. Another image, as suggested by excavations at Oakbank, Loch Tay, Scotland, is one of a raised platform on stilts. The choice of an island as a home remains mysterious, but they may have been used for defence at times of danger, for social display by the wealthy and prosperous, or because islands carried many meanings in the past. Some crannogs could be reached from the nearest shore by means of a causeway built up with stones, or a wooden gangway built atop raised piles, but most were probably accessed by boat.
Late Mesolithic hunter-gatherers are known to have occupied constructed lakeshore platforms in central and northwest Ireland at c.4500 BC. Neolithic crannogs are also known, notable in Scotland. The islet of Eilean Domhnuill, Loch Olabhat on North Uist may be the earliest crannóg, dated to 3200-2800 BC in the Neolithic period. Most crannógs were in use from the Iron Age through to the early Medieval period, at about the same time as the brochs, the wags, duns and the larger roundhouses. In Ireland, most crannogs date to the early medieval period, when they were the island dwelling places of kings, lords, prosperous farmers and occasionally socially marginalised groups.
The largest concentration of crannógs in Ireland are found in the lakelands district of the midlands, the north west and Ulster. The highest concentrations of crannógs (in Scotland) are found in several lochs within Dumfries and Galloway region, although many have been found in the highlands as well. In the Grampian Highlands a well known crannóg was built by the Burnetts of Leys, whose family thence moved nearby to the present 16th century Crathes Castle.
A crannóg dating from around 500 AD still stands in a lough in Loughbrickland, near Banbridge, County Down, and another can be seen in Llangorse Lake in the Brecon Beacons National Park, built c889-893 AD.
A variant of the crannóg was the island roundhouse. Built on a small, rocky island in a lochan and usually reached by means of a causeway, these are extremely common in the Western Isles. The visible remains are most often those of a dún, although there are examples of full broch towers occupying some sites. Not many have been excavated, but the majority of those that have been show earlier occupation underneath the visible remains. Dún is the gaelic word for fort, and a number of Scottish castles use 'Dun-' as a prefix.
The construction of the prehistoric crannóg began on a small island or shoal that was located within a loch or marsh. This rise was surrounded by a circle of oak piles with axe-sharpened bases that were driven into the bottom, forming a circular enclosure of about 200 ft. in diameter. The piles were then joined together by interlaced branches and wattle. The interior surface was then built up, first with wooden logs, then with branches and rocks, clay, peat, and other earthen materials. At the center a large stone hearth was built with large flat stones, and a wooden home was constructed around it. Sometimes multiple homes were built on a single crannóg.
This prehistoric fortification was occupied by a family or tribe, and access was often achieved by means of dugout canoe. However, many were connected to shore by timber or stone causeways, sometimes lying just beneath the surface of the water concealing them from potentially hostile intruders. The bones of cattle, deer, and swine have been found in excavated crannógs.
There is an example of a reconstructed crannóg at the "Scottish Crannóg Centre" at Loch Tay, Perthshire.