Early Christian Ireland

Early Christian Ireland began emerging from a mysterious decline in population and living standards that lasted from about 100 to 300 CE. During this period, known as the Irish Dark Age, the population was entirely rural and scattered, with small ringforts serving as the largest centers of human occupation. These ringforts, of which around 40,000 are known and possibly as many as 50,000 existed, were primarily farm enclosures for the well-to-do and often included souterrains—underground passages used for hiding or escape.

The Irish economy was almost entirely agricultural, though raiding Great Britain for slaves and loot also played a significant role. Crannógs, or lakeside enclosures, were used for crafting and provided an important economic boost. Contrary to earlier views that medieval Irish farming focused mainly on livestock, pollen studies have shown that cereal farming, particularly of barley and oats, became increasingly important from around 200 CE. Livestock, especially cattle, were highly valued, with cattle-raiding being a major part of warfare. Large herds, especially those owned by monasteries, were common by the end of this period.

During the early medieval period, there was significant deforestation, reducing large forest tracts by the 9th century, though boglands remained relatively unaffected. By 800 CE, small towns began to form around larger monasteries, like Trim and Lismore, with some kings based in these monastic towns. Kings generally lived in larger ringforts, but with more luxury objects such as elaborate Celtic brooches. The period also saw the peak of Irish Insular art, with illuminated manuscripts like the Book of Kells, brooches, carved stone high crosses, and metalwork like the Derrynaflan and Ardagh Hoards.

Politically, the oldest certain fact in Irish history is the existence of a pentarchy in late prehistory, comprising the cóiceda or "fifths" of the Ulaid (Ulster), Connachta (Connacht), Laigin (Leinster), Mumu (Munster), and Mide (Meath). However, this pentarchy had dissolved by the dawn of recorded history. The rise of new dynasties, notably the Uí Néill in the north and midlands and the Eóganachta in the southwest, transformed the political landscape. The Uí Néill, along with their parent group the Connachta, reduced the Ulaid's territory to what are now counties Down and Antrim by the 4th or 5th centuries, establishing the tributary kingdom of Airgíalla and the Uí Néill kingdom of Ailech.

The Uí Néill also engaged in regular warfare with the Laigin in the midlands, pushing their territory south to the Kildare/Offaly border and claiming the kingship of Tara, which began to be seen as the High Kingship of Ireland. This led to a new division of Ireland into two halves: Leth Cuinn ("Conn's half") in the north, named after Conn of the Hundred Battles, the supposed ancestor of the Uí Néill and Connachta; and Leth Moga ("Mug's half") in the south, named after Mug Nuadat, the supposed ancestor of the Eoganachta. Although dynastic propaganda claimed this division dated back to the 2nd century, it likely originated in the 8th century, during the height of Uí Néill power.