EARLY HISTORY
I think that the early Druids may have arrived on these shores in the late Neolithic period.
The passage graves are the most well known of all the ancient structures found in Ireland & correspond to many found abroad. They are therefore considered to have been constructed by people who came up the east coast from Brittany and landed near the river Boyne around 3,300 BCE. Those around the Boyne valley are accepted to be the earliest examples but soon this type spread to adjacent areas & further afield as far as Co. Mayo in the west, Co. Antrim in the north & Co. Waterford in the south.

Incised ornamentation on exterior & interior stones appears to reveal spiritual concepts. Examples of lozenges & triangles are common along with varieties of spirals & curves giving the impression of movement and ritual activity. Carved circles are sometimes accompanied by rays and considering the importance of the sun these may be seen as solar representations.

As these Neolithic people were farmers it is logical to assume that the sun played a vital role in their outlook on life & death as well as in their everyday lives. The emphasis on the sun was so great that the builders constructed these passage tombs to receive the sun at special times of the year.

At Knowth the orientation of the two passages is such that the sun rises & sets directly in line with it at the Spring & Autumn equinoxes - the times of sowing & completion of harvesting.

 

The most striking is the solar orientation of Newgrange which allows the suns rays to penetrate through to the inner chamber during the few days centering on the Winter solstice.

It is impossible to reconstruct the actual rituals which took place within these megaliths however, it seems that regeneration & fertility were principle functions of these structures, as well as the idea of continuity of the community & the initiation of a new period of time connected to the sun's position.
The early Celts may have arrived around this time.

The decorative skills of the Bronze Age people is seen in their metal implements but most striking of all was their artwork in gold. An indication of a belief system can be found in the so-called 'sun-discs' about 20 of which survive, dated to around 2,000 BCE. They consist of rounded discs made from sheets of gold with concentric circles and a stylised cross in the centre. As they have two holes also near the centre it is thought that they were attached to leather or cloth wear. Some also have other radial effects suggesting the suns rays and it can be assumed that these were ceremonial objects, perhaps connected to the solar designs on rocks from the earlier Neolithic time.