The last holdout may have been the Fir Domnann (the Dumnonii Men), possibly one of several British tribes who saw fragments of their number move to Ireland.Coverage here of the Irish tribes therefore is a snapshot, figuratively taken by Ptolemy.
By this time, Cairn G, one of a cluster around Carrowkeel in the Bricklieve mountains, has been erected.
Carefully set into the entrance of Cairn G is a hole that is positioned to let the sun's rays into the inner chamber for a month either side of midsummer, with possible extra use for the moon's rays at midwinter.
Trapped between this land bridge and the ice sheet to the north, the Irish Sea is filled by melt water that forms a vast lake.
At this time, the land bridge is finally submerged beneath the salt water of the Atlantic Sea.
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.