Is it correct to say that both "a water bottle" (i.e. a bottle that can be filled with water) and "a bottle of water" (i.e. a bottle that has water in it") would both be referred to as "buidéal uisce"? If so, is context the only way to distinguish the two?

No, there is no difference in Irish.

After a feminine noun or after a plural ending in a slender consonant, there is a question whether use or non-use of lenition could create a distinction. e.g. buidéil fhíona - wine-bottles, buidéil fíona - bottles of wine. But I think this distinction is not always made because f is often unlenited.

Milk bottle/bottle of milk - buidéal bainne (as shown in Ó Dónall's dictionary). He didn't show the plural, but potentially you could argue that buidéil bhainne would be milk bottles but buidéil bainne would be bottles of milk.

I don't know what context would distinguish the two in English, never mind in Irish. A "bottle of water" and a "water bottle" mean the exact same thing, strictly speaking. The distinction you make between "a bottle that can be filled with water" and "a bottle that has water in it" is very specific to the example of a water bottle, and doesn't necessarily hold when you swap in other nouns. For example, a "mud puddle" and a "puddle of mud" are the same thing. A "mud puddle" isn't a puddle that can be filled with mud.

The reason water bottles are an outlier as an example is that more than one thing can make an otherwise ordinary bottle into a "water bottle".

1. It can be currently filled with water (which is the usual meaning of this kind of construction).
2. It can have been originally sold containing water, and perhaps has branding on it which is associated with water (e.g. fiji, ballygowan or volvic).
3. It may be intended or expected only to hold water, though it could potentially contain something else (e.g. "I snuck vodka into the cinema in my water bottle").

These second and third meanings aren't typical of genitive constructions like this, and are secondary to the original meaning, i.e. "a bottle that has water in it". In other words, the term "water bottle" doesn't actually mean "a bottle that can be filled with water" as you suggest. This is only one of at least three possible meanings, the original of which was the exact same as "bottle of water".

An actual distinction between the two constructions can occur in English, though, I think only if you use adjectives. A "bottle of cloudy water" means the water itself is cloudy, whereas a "cloudy water bottle" is ambiguous. It could mean the bottle itself is cloudy, or it could mean the water inside it is cloudy. This kind of ambiguity is likely the reason English maintains two possible word orders for genitive constructions, which otherwise mean the exact same thing. The word order in Irish creates no such ambiguity because of the placement of the adjective (and in this case because of lenition also)

A bottle of cloudy water = buidéal uisce mhodartha
A cloudy bottle of water = buidéal modartha uisce

A wine-bottle can be empty. A bottle of wine has to have some wine it in.

A wine-bottle can be empty = pessimism
A bottle of wine has to have some wine in it = optimism



Anyway, I understand that this contextual difference can exist, particularly where bottles are concerned. Containers in general, in fact (beer keg, pint glass, champagne flute, punch bowl, soda can). My point was that these kinds of constructions are secondary interpretations which aren't possible for all nouns, and that it may be easier to understand the connection with the genitive in Irish by looking only at more restricted nouns (mud puddle, rubbish heap, chicken soup).

Of course, even these can be forced to have secondary meanings (dry mud puddle, empty rubbish heap, vegetarian chicken soup), but in all cases I think you'd still have to translate to Irish based on the primary interpretation (dry puddle of mud, empty heap of rubbish, vegetarian soup of chicken). Hence, you'd use the same genitive construction regardless of how it's expressed in English.

really hard to remember