Early Christian Ireland was known as the Island of Saints and Scholars. Newly Christian monks created famous monasteries and churches and converted Northern Britain and much of Germany while producing beautiful manuscripts like the Books of Kells and Lindisfarne.
Among the booty brought back from Roman Britain was a Romano-British slave named Patrick whose life is generally seen as marking the transition to the next phase of Irish history.
Patrick was remembered in the subsequent hagiographic tradition for introducing Christianity to Ireland although there is evidence that his work was largely confined to the north of the country and the earliest attested Christian missionary is Palladius who was sent to the Irish believing in Christ in 431AD. The expansion of Christianity in Ireland was marked by the construction of semi-independent monastic foundations although these were ultimately brought under the authority of Armagh. The period was also characterised by the appearance of a new settlement type, the Irish ringforts, which seem to be the fortified dwellings of large dairy farmers and remained in use between the 6th and the 12th century. Several of the earlier hill and promontory forts were re-used at this time and the surviving mythological and semi-historical sagas show that ruling dynasties did their best to associate themselves with surviving ancient monuments such as Ard Macha, Tara and Cruachain in order to legitimise their rule. Iron Age ring barrows, referred to in Irish as ferta, remained in use among the elite for several centuries after the introduction of Christianity and it is possible that many earlier monuments and religious traditions were incorporated into the new faith. The expansion of Christianity also provided a vector for the spread of new technology and the earliest tidal mill identified in western Europe appears to have been built by the monks of Nendrum in the sixth and seventh centuries. New traditions, such as pilgrimages to holy mountains and islands associated with famous Saints, were also adopted from the wider Christian world and important monasteries were founded on some of the remote islands off the west coast including good examples on the Skelligs, Aran Islands, Inishmurray, Inishbofin and Omey Island. In many cases, pilgrimages to mountain-top sites such as Mount Brandon, Croagh Patrick and Slieve League remained popular for centuries, in some cases up to the present day.