Scots in Roman service

[Ti Cl Nero]

I guess more to the point, would of been, would any irish, and yes I know they were not called that then, , been in service to the roman legions.

I think you should really focus, then, on the Atacotti units listed in the Notitia Dignitatum.
Next point is the understanding of Atacotti's identity.
Many authors dealt with this problem, placing Atacotti's origin in Ireland, but also in Northern Britain or Western islands (if you need a list of books dealing with the subject just ask).
Rance, in the article I mentioned earlier (2001), concludes that their origin must be Irish, and that "Atacotti" is a Latin development of the Irish word "aithechthùatha", which, by the way, is not the name of a tribe, but a term menaing something like "subject people", a word generally describing lower status tribes among those inhabiting Ireland - the social and political "loser" groups.
The word describes a status, then, and, as a consequence, a group of tribes ( is not uncommon for Romans to refer to gorups of tribes rather than to single tribes)
Their appearance in Wales in late IV century seems (still following Rance) to be more a pacific Voelkerwanderung rather than a mass migration or an invasion, and Atacotti groups which were settled within the empire may have chosen to be subject to the Romans rather than subject to Irish leading tribes.
Their recruiting, then, may be part of some kind of agreement with Romans, whose details are unknown and must be hypotetically extrapolated by comparing this context with other barbarian settlements inside the empire.
The article by Rance deals with these aspects, and also recruiting is widely dealt with (if you need further details about this article just contact me)

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