Ireland is a country rich in historical ruins, impressive dwellings and places devoted to war and worship. Her earliest considerable monuments are the great passage tombs built in neolithic times.
Their main chambers are built on a principle, that each new generation of children discovers in playing with toy bricks; that two walls of bricks or stones can be piled up, with each level projecting slightly inwards until the tops of the walls are close enough to cap with a final slab. Correctly completed, this technique (known as corbelling) makes it possible to erect firmly fixed structures without using cement. Christian monks were still employing this method over three thousand years later.
From about 500 BC, substantial defensive works appeared all over Ireland. This is most often attributed to the warlike character of Celtic society; broken up into many kingdoms, although some believe that the largest sites may have served ritual rather than practical purposes. These sites, known as hill-forts. are more or less ornate detailed systems of stone walls and earth ramparts, following the contours of a hill to create a town-sized defensive structure. It is still not clear to what extent they were ceremonial centres, permanent settlements or simply places of shelter and protection, capable of holding large numbers of people and their herds during a crisis.
Such questions are difficult to answer due to the fact that many hill-forts were occupied both before and after the Iron Age; for example, an important feature on the Hill of Tara, the so-called Mound of Hostages, is a passage grave. Tara must therefore have a long history as a special place, since, long afterwards, during the early centuries of the Christian era, the high kings of Ireland were consecrated there. An equally impressive evocative site, the mound of Navan Fort near Armagh, is the Emain Macha of Irish myth, from which Conchobar ruled Ulster.
Another type of defensive structure was the ring fort. As its simplest, this formed a farmstead or small settlement surrounded by a drystone wall or earth rampart. Many ring forts had underground chambers that served as storage-pits or possibly, hiding-places in the event of the fort being captured. Well-known examples of ring forts are Staigne Fort, County Kerry, and the Grianan of Aileach, County Donegal, both of which have walls up to five metres high with chambers inside them and steps leading up to the platform on top.