Just a side thing, but these are some remarks from Lars Braesicke's on-line grammar:
Quote:
In older textbooks this (bí and adjective ), despite its common use, is stylistically frowned upon. However, it was not the copula with [the] predicative adjective [that was the issue] , but [rather] the use of attributive adjectives was recommended instead.
Also: Is duine mór é sin = That is a big person (instead of Tá an duine sin mór or Is mór é an duine sin = That person is big).
Quote:
Once it was the rule of thumb that only adverbs could appear with the verb bí, always. ("adverbs with bí, adjectives with is")
Today this only is the case for a few adjectives (see above), while the predicative use of adjectives with bí is possible and has become common.