"The" is what's called a definite article. It means you're referring to a specific thing. "Give me the book" means I'm indicating a particular book, and I want that one.
"A/an" is what's called an indefinite article. It means you're not referring to any particular thing. "Give me a book" means I don't care what book, any book will do.
Irish only has the definite article: "an/na". It does not have any indefinite articles. So if you come across a noun phrase without any article, you'll need to use judgment as to whether it needs to be translated with an indefinite article or none at all.