[The 'Celtic world' was not confined to Ireland and Scotland - as the following demonstrates]
The great water-healing shrines of the Celtic world did not fully flourish until the Roman-Celtic period. In Roman Britain and in Gaul there were a number of curative cults who made regular pilgrimages to the great centres at places such as Bath, Lydney and Source des Roches de Chamalières in the hope of being healed. Not all of these sites were temples in the grand style of Bath. Some were merely ornamented pools based around springs or natural clefts through which healing waters ran. Pilgrims flocked to these centres, some bearing wooden or clay representations of themselves in their distressed state believing that the deity would replace these with better models and so cure them

In earliest times, there were local spirits or forces that performed healing in return for supplication and worship. Over the years, recognisable deities began to evolve, largely due to classical influences. In northern Britain there were a number of identifiable aquatic deities, the most important being the goddess Coventina who presided over a site near Carrawburgh. Similarly Verbeia was worshipped at the rivers Wharfe and Tees while a number of other water-nymphs were the object of veneration at sites along the Humber. A name that occurs time and again is that of Latis. This might have been a generalised term since it means ‘goddess of the Pool’. Another virtually nameless incarnation of the same goddess may be found at the healing springs at Buxton where the name Aquae Arnemetiae means ‘the waters of the goddess who lived in the sacred grove’ – a concept that united two sacred Celtic sites

Water was associated not only with healing. In the Celtic mind it was also associated with fertility and renewed life. After all, water was necessary to make crops grow and to restore vitality to parched earth – and people. Some deities might have combined the attributes of healing and fertility, becoming important entities closely connected to the source and creation of life. Along the River Boyne in Ireland, a number of early Celtic tribes believed that they had sprung from the loins of the river goddess Bóinn. The mating of the supreme god Daghdha and the river-goddess Bóinn [in Ireland's Celtic mythology] is symbolic of the union between the fertile land and the river-water that brought about growth and irrigated crops. The marriage between a river-spirit and a tribal god was not uncommon among the Celts. The river ensured the well being, fertility and rejuvenation of land and people. At many sites, such as Bath and Buxton, offerings of money and jewellery are to be found – symbols of hope and requests for good health and well being

For the ancient Celts, water was a powerful symbol and the number of water-shrines and sacred pools that flowered during the Roman occupation of Western Europe bears testament to this. Although the major aquatic shrines date from that period, there is no doubt that springs and pools were being enclosed as sacred sites throughout Europe long before the Romans arrived

Ground, bog, lake, mountain, grove and pool – all were the embodiment of the landscape as well as the home of the gods. As such they were places of veneration, areas that needed to be ‘sectioned off’ from the rest of the world. Surely this is the origin of temples and churches even today.

From Complete Guide to Celtic Mythology by Bob Curran