The elements
In the lives of the saints and other religious writings as well as in the lay literature, evidence has been found that various natural objects were worshipped by the ancient Irish. But these worships were partly confined to individuals or to the people of certain districts, each individual, or family, or group having some special favorite object. There is no record of the universal worship of any element, there is reason to believe that it was not mere material object they worshipped but a spirit or genius supposed to dwell within it.
The Sun
That the sun was worshipped in Ireland at least partially, like some other natural objects, is made certain by several passages in our ancient literature. St. Patrick plainly intimates this when he says in his Confession speaking to the Irish that all who adore the sun shall perish eternally. This is a contemporary statement for the saint is evidently denouncing a practice existing in his own time.
There are more specific account in Cormac's Glossary but this entry is from four centuries later and the records are not from contemporary custom but one existing long before the time of the compilation of the glossary. It states that Indelba (images) was the name applied to the altars of certain idols and that these altars were so called because "the pagans were wont to carve on them the forms" of the elements they adored. In this case the sun. One of the three last Dedannan kings of Ireland was named Mac Grena "son or devotee of the sun" since he worshipped the sun as his God.
Fire
That fire was worshipped by some of the Irish is clear since it's stated in the Tripartite life that Leagaire's druid accused St. Patrick of having fire for a god, which shows that the idea of fire-worship was familiar.
It's already known that fires were kindled by the druids at Ushnagn in honour of the god Bel, and that fire played a prominent part in certain pagan festivals.
Many of these quite harmless fire-ceremonies have descended to our own time, some signalizing 1st May, and some the eve of 24th June, when people light open-air fires as soon as the dusk comes on so that the whole country is illuminated.
Wells
The worship of water, as represented in wells, is often mentioned. The Tripartite life and Tirechan in the book of Armagh relate that St. Patrick in his journey through Connaught came to a well called Slan (means healing), which the heathens worshipped as a god, believing that a certain prophet and druid had caused himself to be buried under it in a stone coffin to keep his bones cool from fire that he dreaded, "for he adored water as a god but hated fire as an evil being".
Elemental oath
No doubt this ancient elemental worship was the origin of the very general pagan Irish custom of swearing by the elements, or in other words giving the elements as guarantee, an oath witch was believed very dangerous to violate, as shown by the fate of Laegaire.
Laegaire was king of Ireland in the time of St. Patrick and in an attempt to extract an treasure from Leinster he was defeated and taken prisoner by the Leinstermen, but was released on taking the usual oath, giving as a guarantee (i.e swearing by) the moon and sun, water and air, day and night, sea and land, that he would never try and demand it again.
But in open violation of his oath he invaded Leinster anyway (A.D. 463) and "the sun and wind killed him because he had violated them".
