"He is not that professional" is what I put, because of the "sin". Apparently that is wrong. Why is "sin" there?
Because the sin is part of the pronoun, along with sé. It expresses the pronoun "that".
Sounds to me like the stress is on the second syllable of "proifisiúnta" in the audio for this example. But in an earlier example with "an bhean phroifisiúnta" the first syllable sounded stressed. Is this stress shift a consistent thing, or does stress vary in this word? Or maybe it's an error, or I'm just hearing something wrong?
I wrote "This is not professional," rather than "That"; is there a distinction to be made, or should my answer be accepted?
"sin" = "that", "seo" = "this". Not a big difference, still it is. In other languages "This" would be interpreted as something close by, while "That" is something out of reach.