It is unknown when the Celts came to Ireland, but it is likely they brought the use of iron with them. The use of iron marks the start of the Iron Age. It is known that by about 300BC, the use of iron and Celtic culture was widespread in Ireland. The Celts lived in ring forts, hill forts, promontory forts and crannógs. It is thought that only the richer families and settlements lived in crannógs. These were man-made islands in the middle of lakes with houses on them.