Neilson: líon beag: an Mhí-Cabhán-Lú. Comh-shloinne le Mac Néill ach go bhfuil nasc Lochannach ann. Tá Njall (Lochlannach) comhchiallmhar le Niall.
Quite numerous: Meath-Cavan-Louth etc. Ir. Mac Neighill. Derived from Norse first name Njall which is from Irish Niall. Popular English first name Nigel is cognate. So this name is basically the same as Neill & Mac Neill, q.v.
Very numerous: Ulster, especially N Down. Ir. Mac Neighill. The well-known English name is present here; ironically, it derives ultimately from Irish Niall which is discussed under Neal and Neilson. DBS & DOS.