Real knowledge begins with two Celtic invasions, that of the Goidels in the later part of the Bronze Age, and that of the Brythons and Belgae in the Iron Age.
But an earlier wave of Celtic invasion represented by the Goidels had passed westwards along the valleys of the Usk and Wye, leaving traces in place-names (e.g.
ST KENTIGERN, or Mungo ("dear friend," a name given to him, according to Jocelyn, by St Servanus), a Briton of Strathclyde, called by the Goidels In Glaschu, " the Grey Hound," was, according to the legends preserved in the lives which remain, of royal descent.
The population of the north shore of the Solway Firth at the beginning of the 5th century were probably either Picts or Goidels or a blend of both, and naturally hostile to the Romanized Britons.
At any rate it was not until well on in the Bronze Age, perhaps about 600 or 500 B.C., that the Goidels, the first invaders speaking a Celtic language, set foot in Ireland.