Can Johnson be an Irish name?

Answer 1

No,"son", indicates that it's an English name meaning son of, Irish names would have "Mac/Mc", for the Irish word "Mac", son.

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I respectfully beg to disagree with the above answer. Johnson can be Irish (as well as Scottish, Scots-Irish & English). Some Johnson's had more Irish sounding names but there was a time when many Irish names were Anglicized.

From this source:

http://www.irelandseye.com/aarticles/features/tracingi.shtm

The process of Anglicisation, combined with the fact that before the twentieth century illiteracy was widespread, means that many Irish surnames can be spelt in a variety of ways. Consider, for example, the surname Johnston. Variants of this include Johnstone, Johnston, Johnson, MacEion (son of John), MacOwen, MacKeon, and MacKeown. The surname Kearns can be variously rendered as Cairns, Carns, Cearnes, Kerns and Kearnes.

There's more at the attached Wikipedia link:

There's more if you have time to do more research.

The problem here is the same problem with many surnames that have been modified over the years as the families immigrated or invasions occurred. Names can change in what looks like a dramatic way, such as McKeown to Johnson,or subtle ways, Johnston to Johnson but the basic roots remain the same.