In Ireland, Williamson is almost exclusive to Ulster and is common in counties Antrim, Derry, Armagh and Tyrone; most will be of Scottish origins. The personal name William derives from the Old German Willihelm and when introduced into Britain by the Normans, it became the single most popular personal name in England and remained so until it was superseded by John. It gave rise to a host of surnames including Williamson and Williams but by far the most common was Williams. Williams was never common in Scotland which retained the longer Williamson. This is very common in the Lowlands. There were MacWilliams or Williamsons, a sept of Clan Gunn, who descended from a later chief called William. There were also Williamsons in Caithness, a sept of Clan Mackay. Quoted from "The Book of Scots-Irish Family Names" by Robert Bell, pg 250.

Link to Gunn Clan/ Sept Williamson

More Gunn info/Williamson