Laigain refers to a group of peoples who are referred to as the the Laiginians.
The Laiginian colonization is believed to have taken place sometime about 300 B.C., and are believed to have come from the northwestern region of Gaul, later Normandy. They are mythologically referred to as the Tuatha De Danann. Their name association with Laighi, the ancient name for Leinster, suggests that this was where they first settled. Eventually, they extended their power to Connacht, and in the process forced the Firbolg tribes into the remoter parts of the province. The remains of many great stone forts built by the Firbolgs in their defense against the Laigain tribes can still be seen in remote areas of western Ireland. Within a few generations the Laigain tribes had established themselves in Connacht, where in County Sligo their descendants include the O’Haras, O’Garas, and others.
Source: http://www.thecore.com/let-ros/plavin/lavin.html
The ancient Laigin or Dumnonii group moved from the western region of Normandy as the Roman built up pressure on Gaul about 100 B.C. The Laigin settled first in southern Britain and then in Ireland. The Ui Neachtain (Naughton) are said to belong to the Laigain group, later living in the territory of the Ui Maine.
Source: http://miso.wwa.com/~naughton/nau_hist/naughton.html