What the plural of sgian dubh?

[Calgacus]

I have a Gaelic dictionary to hand (Malcolm MacLennan) and it gives

sgian, n.f. gen. sgine and sgidhinn, pl. sginean, a knife; sgian-pheann, penknife; sgian-luthaidh, clasp-knife; sgian-bhuird, table knife...

So the plural of 'knife' is 'sginean'.

What I don't know is that when you make a noun plural, if the adjective has to mutate to agree. (It seems silly in English because 'black' is 'black' whether there's one black thing or many, but in some languages the adjective has to take a plural form.)

In gaelic, the adjective does not mutate.