It's strange: am Faclair Beag defines sgoinneil as
1 trim, well-made 2 careful 3 neat, tidy 4 cool, groovy (colloq.)
Careful, heedful. 2 Attentive. 3 Efficacious, producing a good effect. 4 Neat, trim, tidy. 5 *Decent, tasteful. 6(DMy) Energetic, active, bustling. Duine sgoinneil, an active brisk man.
I don't have access to a copy of Mark, to see what he has to say, but without his input, I wonder whether this is a word that has drifted in meaning over the years, and whether the meanings brilliant and wonderful are fairly recent, unless they come under am Faclair Beag's "cool, groovy".
sgoinneil, -e a 1. neat, tidy, careful, trim 2. brisk, active, energetic, lively □ tha e a’ dèanamh adhartas sgoinneil he is making brisk progress □ tha na dealbhan sgoinneil the illustrations are lively 3. (colloq.) great, brilliant □ bha na taighean-seinnse sgoinneil anns na làithean ud the pubs were great in those days □ làithean sgoinneil! great days!
So Mark does recognise the colloquial use, but not with wonderful. I think the examples he gives are exactly where I would use wonderful in English.
Of course both brilliant and wonderful have themselves diverged from their basic meaning here. It is an area where we get a succession of words being used ephemerally. A look in the HTE shows that this sort of usage cannot be categorised easily. I think it is just too vague. I found various words used with this sense in specific contexts, but no actual category. I suspect there is a whole litany of words in any language that have been used briefly to express that fact that something is very good, in a totally unspecific way, and that sgoinneil and wonderful are just current examples, though there may be huge variation both geographically and according to the age of the speaker.
Literally it means 'that is in me'.
This is one of the standard ways of saying 'I am a ...' whether it is a chef or an idiot or any other sort of thing you can be. It is not used with adjectives ('I am clever') and it is not used with definite nouns ('I am the chef').