Jamie wrote:
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?
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 mhodarthaA cloudy bottle of water =
buidéal modartha uisce