silmeth wrote:
You cannot use
bí to say that someone ‘is a beautiful woman’, you need copula for that (as you cannot say eg.
*tá tú bean álainn, only
is bean álainn tú or
(is) bean álainn atá ionat).
So I think it’d be
ceapaim gur bean álainn tú.
But I cannot find anything about copula in indirect speech in the GnaG, at least
Wiktionary confirms gur is used for ‘that is’.
Possibly also
ceapaim go bhfuil tú i do bhean álainn (or
ceapaim go bhfuilir id’ mhnaoi álainn for archaized Munster :P), but I am not sure what subtle differences in meaning this version has. ;-)
EDIT: And I’m late, cause Breandán answered in the meantime. Well, I’ll still post my answer.
Your answer raises other points not covered in my answer.
Let us compare the two forms:
A.
Is bean álainn thú. "You are a beautiful woman."
B.
Tá tú i do bhean álainn. "You are a beautiful woman."
A is a statement of fact.
B is a state. There is a subtle nuance of impermanence. If said to a very young woman, you might be implying that she is no longer a little girl. If said to an older woman, on the other hand, you might be felt to be implying that she won't always be a beautiful woman. ("Seal go rabhas im' mhaighdean shéimh, anois i mo bhaintreach chaite thréith ...")
Just as the English "You look beautiful this evening!" may invite the reaction "Does that mean I don't usually look beautiful?", be careful using B and especially be careful what adverbs you put with it.