Donoghlane says that "special" can have negative connotations - I maintain that "special" itself doesn't have negative connotations, but that it can be used sarcastically, which is where the negative connotation comes from.
The people who volunteer with the Special Olympics (Oilimpicí Speisialta) aren't using "special" in a negative way - if it's a euphism, it doesn't have any inherent negative quality. It only becomes a negative term when it is use to refer to a persons handicap in a sarcastic way.
The issue is getting into which ones you accept. If you accept "ye," do you accept "y'all?" What about "youse" or "you guys"? What about distinguishing for those dialects in England that still use "thee" and "thou" for singular 2nd person? It's just easier to accept only the standard.
"He is important" is Tá sé tábhachtach. "He is an important man" is Is fear tábhachtach é.
tábhachtach and speisialta are adjectives - you use the copula when you are connecting two nouns (or a noun (fear) and a pronoun(é)), not when you are using an adjective to describe a single noun.