Several other families now using the name of Thompson etc.. in Ireland were originally of another surname entirely.
Mc Tavish and Mc Avish are two such instances. People of these names have changed their names into Thompson on occasion, as documented by the Registrar-General in relatively recent times.
Most of the name are likely of settler origins in Ireland, found mainly in the province of Ulster. Note that the name of Thomson has been said more often to denote a Scottish origin when found in Ireland. When spelled as Thompson it origins are more likely English according to sources in the past.
The Thompson family of Rathnally and Ross, Co. Meath, descend from John Thompson of Muckamore.
The Thompson family of Clonfin, Co. Longford, descend from the first of that family to settle in Ireland, one Capt. William Thompson, b. 1655, who came here with King William II in 1688.
The 1890 birth index finds 317 births of the name, mainly in Antrim, Down, Armagh, Londonderry, Dublin, Fermanagh, and Longford.
The census of 1659 finds Thompson in Wexford, Meath, Armagh, and Clare. The spelling of Thomsone at that time is found in County Donegal.