The present continuous tense that we use all the time in modern English (Is/are ___ing) is just that, modern. Most other older languages don't use it, but mean the same thing. Go back to the King James Bible and what do you see? Not "Forgive them father for they don't know what they are doing," but "Forgive them father, for they know not what they do."
Irish and English both have separate forms for the present continuous and the simple present - this may be confusing if you're used to translating from a language that doesn't differentiate between the present simple and the present continuous, but when translating between two languages that have both tenses, such as English and Irish, the two tenses are quite distinct, and you can't translate from the simple present ("she drinks"/ólann sí)in one language to the present continuous ("she is drinking"/tá sí ag ól) in the other.
Well, "Whisky" is just an anglicization of the Irish / Scottish word "Uisce", meaning water. (Whisky usually refers to Scotch, Whiskey to the Irish variety). You're correct that the Irish name for the drink is "Uisce Beatha", which means "Water of Life".
Is it true that the verb doesn't change form between singular and plural nouns?
Yes. Think of it this way: Ólann an cailín (the girl drinks) is similar to Ólann sí (she drinks), so therefore Ólann na cailíní (the girls drink) is similar to Ólann siad (they drink). When you consider it this way, you can see that there is no need to change the verb.
When do you use "an" and when "na" and should the word after be in singular or plural. I'm so confused, cause sometimes it says "na" but then the following word in singular. What is like defining it?
From the Tips & Notes for the "Plurals" skill:
There are two forms of the definite article in Irish.
An is used for singular nouns and is translated as the in English. For example, an buachaill means the boy.
Na is used for plural nouns and is also translated as the in English. For example, na buachaillí means the boys.
At this stage of the course, if the noun is plural, use na, if the noun is singular, use an.
(When you get to the Genitive case, you will encounter examples where na is used with singular feminine nouns, but you won't get to that for a while yet).