What is this "go"?
What is the grammatical name and the function of the "go" word in expression like "go deas", "go dona", "go héasca", "go soiléir"?
"Go" before an adjective does one of two things. Properly, it converts an adjective into an adverb, e.g. sollúnta "solemn" - go sollúnta "solemnly". In a number of cases, however, it is conventional to use it with some adjectives in a predicative position, in which case the adjective remains an adjective, e.g. Táim go dona "I'm terrible" or Tá sí go maith "she's well". Some don't do this though, e.g. Táim tuirseach "I'm tired", so you just have to get used to the ones that do.
Just for future reference, 'go' also is used as a preposition that means 'to' (as in physical movement). It's used with destinations that do not have a definite article, e.g. Táim ag dul go hÉirinn "I am going to Ireland." (Note that "go" here adds an "h" before a vowel and does nothing to a consonant.) Contrariwise, you use "go dtí" when a definite article follows, e.g. Táimid ag dul go dtí an scoil "We are going to the school."
Also, it marks the subjuctive mood. The best example is "Go raibh maith agat", which literally means "May you have good" or "May good be upon you". That jussive "may it be" is the subjunctive mood that is denoted by "go raibh".
Finally, it is a conjunction that marks reported speech and subordinate clauses for present, conditional and future tenses. It corresponds loosely to "that" in English, in the context of such a sentence as "She told me that Seán sings often." This would be translated to "D'inis sí dom go gcanann Seán go minic." Note that it introduces the reported speech, and eclipses the following word if applicable. In past tenses you would use "gur" instead. The negative versions are "nach" and "nur", corresponding to "go" and "gur".
This is a lot of information, but I figured getting it all clearly out would be fairly helpful. I hope everything I've said is accurate; if not, I'm sure someone will come along and kindly correct me.
Thank you, I'm saving this post for future reference ;)
I suspected that "go" was used to transform an adjective to an adverb, but then I stumbled upon its predicative use ("Tá sé go dona"), and "go dtí" and I got a bit confused. I hope the course maintainers will add some information on this little but important word in the final version. So... go raibh maith agat!