When can you use 'a' or 'an' before an uncountable noun?

What is correct: coffee/a coffee; ice-cream/an ice-cream?

The rule applies, the indefinite article is used only before countable nouns. If we use it before "water", "coffee" or "ice-cream", this means that in this case they are used with a different meaning and/or somehow have turned into countable nouns. How is this possible:

When we talk about the whole, it is uncountable: There is hair on the floor.(collective noun)
When talking about a part of it, that is countable: There is a hair on the floor. (only one strand)

The material something is made of is uncountable: The table is made of glass.
If the object is called the same, it is countable: There is a glass on the table.

Liquids are uncountable: I like beer.
A pre-determined quantity is countable: We'll have two beers, please. (two bottles of beer)

Examples of words that are uncountable in one meaning and countable in another:

glass	-	material	a glass	-	used to put liquids in and to drink from
beauty	-	a quality	a beauty	-	a beautiful woman
paper	-	material	a paper	-	a newspaper
drawing	-	the act of using a pencil to create a drawing	a drawing	-	the product of drawing
iron	-	material, a kind of metal	an iron	-	an appliance used to smooth out wrinkles on clothes
football	-	a type of sport	a football	-	the ball used in this sport
hair	-	the collective hairs on our head	a hair	-	a single strand of hair
medicine	-	a science	a medicine	-	medication, a pill or syrup

More on the subject:
Articles - a, an, the
Uncountable nouns in English