An- (often pronounced ana-, though always written as an-) is affixed to adjectives and means "very". It is not the same as the definite article an. e.g. Bhí an scannán an-mhaith ar fad. "The film was very good altogether."
Im pretty sure its a particular area of irish grammar thats really confusing and difficult and it has a name but i forget it cos i mostly zoned out in confusion when we were tought it but i think its that when a word starts and another word ends with a combination of these two letters a(n) (t)e d,n,t,l,s they dont add in a h for it...or maybe te is masculine and im pretty sure those dont get a h. It's quite confusing but there is a rule about it