The early Druids may have arrived at the Irish shores in the late Neolithic period. The passage graves are the most well known of all the ancient structures found in Ireland, and 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 and further afield as far as County Mayo in the west, County Antrim in the north and County Waterford in the south.
Incised ornamentation on exterior and interior stones appears to reveal spiritual concepts. Examples of lozenges and triangles are common, along with varieties of spirals and 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 and 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 and sets directly in line with it at the Spring and Autumn equinoxes, the times of sowing and completion of harvesting. The most striking is the solar orientation of Newgrange which allows the sun's 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 and fertility were principle functions of these structures, as well as the idea of continuity of the community and 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 are 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 stylized cross in the center.
As they have two holes also near the center, it is thought that they were attached to leather or cloth wear. Some also have other radial effects suggesting the sun's rays, and it can be assumed that these were ceremonial objects, perhaps connected to the solar designs on rocks from the earlier Neolithic time.
The practice of burying valuable objects developed in the Bronze Age with hoards of implements and artwork being deposited often in bogs, pools & lakes where retrieval would have been very difficult.
It is likely that this custom along with similar customs in Europe was magico-religious in its purpose and that these objects were offerings. In the later Bronze Age, Ireland saw the appearance of more new settlers, who brought with them the refinement of metalwork and two new types of dwellings: the "crannóg" that means "lake dwelling", and the elaborate hill-fort. The hill-forts, many of them built enclosing earlier tombs, were constructed on sites dominating the landscape.
The rapid development in metallurgy at this time, most notably the smelting of iron, seems to indicate the growth of more structured and organized societies and the increase in weapons found by archaeologists may also indicate more military power. It appears that with the coming of the Iron Age, there was more conflict amongst groups in Ireland, particularly between those in the east and those in the west.