"I want a glass of water."
Translation:Tá gloine uisce uaim.
You’d use the same sentence. They’re two different uses of the same genitive form, so ambiguity is possible without context.
I think could say 'tá gloine le haghaidh uisce uaim' to avoid the ambiguity, i.e. 'I want a glass for water', if, for example, you had been given only a wine glass.
I tried to use the genitive article: "Tá gloine na uisce uaim." IF uisce is indeed considered genitive in this sentence, why is it wrong to use "na"? Maybe I'm doing this too early in the morning, but I can't get my head around it right now.
There is no definite article in the English sentence, and you would not need one in Irish either.
Oh, that makes sense. So you can use genitive without the article. That was probably seen in the genitive lesson, but I forgot.
Your sentence would translate as "I want the glass of the water"; the meaning is different.
Because it is an inherent part of a genitive construction - or, if you prefer, English uses "of" to mark the genitive, and Irish uses the tuiseal ginideach.