What the plural of sgian dubh?

[Calgacus]

Thanks!

So sginean-dubh?

I take it that the noun does not have initial mutation in this case?

You've reached the limit of my knowledge of Gaelic, but going from Nathan's Wikipedia quote above...

The plural is most commonly sgian-dubhs (in its various spellings) but sgians-dubh is also occasionally encountered. The proper Gaelic plural forms sg(e)inean-dubh or sgianan-dubh are only rarely encountered in English usage.

...you appear to have a couple of choices.