"That is really dirty."
Translation:Tha sin gu math salach.
Because Scottish Gaelic has two different to be verbs, the copula and the bi verb. And they serve different purposes.
Sin in the meaning that is works only as a copula – the one for defining or equating things (it says what or who that is), not describing them (it never says what that is like). And that is really dirty tells you what that is like, not what it is. For that you need the verb bi (tha, a bheil, chan eil, bidh, bha, etc.). See my answer to similar question in "Tha sin dona" thread.
Technically: you only use sin and seo in the meanings of that is and this is only when the predicate is a noun phrase (a dog, the woman I met, my father, a huge pile of dirt, etc.), and you need to say tha sin… when the predicate is an adjective (dirty, really dirty, nice, huge, etc.) or a prepositional phrase (on the table, in the fridge).