The Market Cross, as it is known locally, is in fact a composite object, assembled from the scattered fragments of two distinct sandstone crosses, both dating to the mid-twelfth century. The base and shaft belong to one cross; the head to another. That the two sit together at all is largely an accident of exhibition logistics, first brought together for the Great Industrial Exhibition held in Dublin in 1853, then returned to Tuam, and not actually erected in the town's Market Square until 1874. In 1990 it was moved again, this time into the cathedral, where it has remained.

The inscription on the square tapering base offers one of the more evocative fragments of medieval patronage in the west of Ireland. It reads, in part, 'Prayer for Turloch O'Conor ... of Iarlath by whom was made,' linking the cross to Turlough O'Connor, the twelfth-century king of Connacht, and to the see of Saint Iarlath, the patron of Tuam. The base itself, measuring roughly 1.65 metres long and 0.65 metres high, carries two pairs of figures carved in high relief on opposing faces, along with two small circular vertical holes whose original function is not recorded. The shaft, nearly three metres tall, is carved in the Urnes style, a late Viking ornamental tradition characterised by sinuous, interlacing animals and plant forms that was adopted widely in Ireland during the Romanesque period. Both base and shaft are covered in this lightly incised decoration, giving the stonework an almost textile quality up close. Before all of this, the base had a more workaday setting: it originally stood in what was called 'the old shambles,' a butchery area on the west side of Vicar Street in Tuam.