Evidence suggests that the first major phase of Crannog occupation occurred in the Late Bronze Age (c. 1000 BC), they may have been occasionally used through the Iron Age (c. 600BC - 400 AD) and in some cases, were still being occupied as late as the 17th century.
Crannógs were often artificial islands (although sometimes natural islands were used) on which people built houses, and lived in relative security. They were constructed by placing layers of stone, brushwood, tree trunks and even old dugout canoes, on the lake bed. These were revetted by wooden pilings and the platform was covered with a layer of earth or sand. On this, the inhabitants built their thatched houses of wattles and thatch, and surrounded themselves with a protective timber palisade fence.
These artificial islands were generally approached by boat, or occasionally by various types of causeways or bridges.