At some point after about 500 BC, there were certainly arrivals by Indo-European Celts (and perhaps even as early as 2000 BC), and they remained fully independent as Ireland was never conquered by the Romans.
Instead the Celto-Irish helped to hasten the end of Roman control over Britain by constantly raiding the British coastline, capturing slaves and booty.
The later high kings were nominally in charge but in practice, descended as many were from the prominent U Neill clan, there were always stresses and strains with the other regions.
For the earlier high kings, the title was more of a ceremonial one, and never implied political control of the whole country.
By this time, Cairn G, one of a cluster around Carrowkeel in the Bricklieve mountains, has been erected.
Ireland was never politically united enough to translate its religious and cultural influence into political power.
The most striking feature of pre-Ptolemy Ireland are legends of the island being divided in half between north and south.
Post-Ptolemy, the four or five kingdoms with which we are familiar began to appear (Connacht, Laigin, Mide, Munster, and Ulaid), but each of these kingdoms were composed of multiple tribes.
The passage tomb there is built around this time according to archaeological estimations, clearly constructed by people who have a sophisticated understanding of the motions of the sun, moon, and stars, thanks to the tomb's alignment and the lunar map found inside it.
Gaelic Ireland There exists a very small window through which to view the tribes of Ireland (those which largely pre-date the Roman presence in Britain).
Mountsandel in Coleraine in the north of Ireland is the oldest known site of settlement, with remains of woven huts, stone tools, and food such as berries and hazelnuts being discovered there by archaeologists.