Tall cross at Monasterboice, ninth century

A HISTORY OF IRELAND IN 100 OBJECTS:When hurlers and Gaelic footballers describe their ultimate ambition, they often use a simple shorthand: "A Celtic cross". Since the late 19th century, the Gaelic Athletic Association has used a high cross for its logo and for All-Ireland medals. The modern use of the cross as a symbol of Irish achievement goes back at least to the 1853 Irish Industrial Exhibition, in Dublin, which displayed them as "fine monuments of the artistic skill and devoted piety of our Celtic ancestors".

The crosses are so deeply embedded in the Irish imagination that it seems almost sacrilegious to ask why they were made in the first place. There was no native tradition of building in cut stone, so the appearance of high crosses in the eighth century was a major cultural innovation. So, as we have seen, was the idea of depicting, in a relatively realistic way, human subjects and stories. The crosses are, indeed, unique to Ireland and Irish-influenced Scotland. They required a huge investment of skill and resources and, as Roger Stalley has put it, “It is hard to believe they were undertaken for purely altruistic or religious motives.” And yet they were erected on a very large scale: about 300 of them survive, of which 100 are decorated with carved images.

The crosses were undoubtedly used as gathering places for prayers by monks and pilgrims, but their scale and complexity far exceed this basic function. This cross, from Monasterboice in Co Louth, is almost seven metres tall, and every available face is covered with elaborate carvings of a dazzling variety of scenes. The east face alone has Christ walking on the water, King David, St Anthony tempted by demons, St Paul and St Anthony killing a devil, an angel shielding three children in the fiery furnace, and images of Elijah, Moses, Abraham and Isaac, David and Goliath, and David (again) killing a lion.

Some crosses are inscribed with the names of kings or abbots, suggesting that they functioned as potent symbols of the power and status of these dignitaries. Like so many objects from pre-Christian Ireland, part of their function is to claim territory and mark boundaries. It is striking in this regard that the crosses are highly individual, with distinctive styles associated with different regions.

The basic form is common to all of them: a pyramidal base, a rectangular shaft culminating in a capstone, and a large circle enclosing the arms of the cross. This circle may be intended to represent a halo around the figure of Christ, but it can also be seen as a continuation of a much older Irish tradition of representations of the sun.

One way of looking at the crosses, though, is that they represent a new assertion of biblical Christianity in the face of a new pagan threat. By the time the cross-builders were at their most active, that threat was all too real.

Where to see it Monasterboice, Drogheda, Co Louth, 041-9837070; a replica is on display at the Irish High Crosses exhibition at the National Museum – Decorative Arts History, Collins Barracks, D7; museum.ie