"My parents are lawyers."
Translation:Is dlíodóirí iad mo thuismitheoirí.
What is it that makes the "iad" necessary? "I am a man" is just "Is fear me", so why not also just "Is dlíodóirí mo thuismitheoiri"? Does the copula always require a simple pronoun and if the predicate/object is more complex, the corresponding pronoun has to be inserted?
??? It has nothing to do with characteristics. When you are categorizing your parents as lawyers, or as people, you must use the copula.
"(noun) is (noun)" -> is (copula)
"(noun) is (adjective)" - bí
Is dlíodóirí iad mo thuismitheoirí - "My parents are lawyers"
Is daoine iad mo thuismitheoirí - "My parents are people"
Tá mo thuismitheoirí sláintiúil/tanaí/ramhar/greanmhar/sean/flaithiúil - "My parents are healthy/thin/fat/funny/old/generous"
While "healthy/thin/fat/funny/old/generous" might be considered characteristics, "cold", "wet", "too hot", "here", probably aren't "characteristics" -
Tá mo thuismitheoirí fuar/fluich/ro-the/anseo - "My parents are cold/wet/too hot/here".
You're jumbling up a couple of different constructions and ending up with a just jumble of words.
You can use tá with the preposition i to replace certain copular constructions - they aren't entirely equivalent, as they have slightly different implications, but it is possible to say something like Tá mo thuismitheoirí ina ndlíodóirí. Where the subject is a pronoun, you can change the emphasis slightly and say something like dlíodóirí atá iontú. Theoretically, I suppose you could say dlíodóirí atá i mo thuismitheoirí, but it's not something I've encountered.
You'll note that these use either a noun (tuismitheoirí) with a preposition, or a prepositional pronoun (iontu) - not both.
The copular construction uses a pronoun (iad) as a sub-subject, but the actual subject is mo thuismitheorí. If you're not using the copula, you don't need a sub-subject, so you don't use iontu.