Presumably we're looking at a race coming up from Iberia (common theory) and for some reason settling in Sligo around Knocknarea! If only they'd known that the weather would turn out to be so shite there today :-)
The rock cut passage tomb on Croghan has provided a radio carbon date of 6800bce and a similar date has been found in the tomb at Primrose Hill on the slopes of Knocknarea.
Carrowmore has provided many dates in the region of 5500 bce.