How do I send documents to CAO

I'm currently looking to apply for university in the Republic of Ireland but I don't know what I'm supposed to include in my application. I have photocopies of my AS levels and my GCSE's but I haven't the slightest clue on what or how to send documents away. Do I include a return address on the envelope? do I have to include my CAO number in the letter? or are the documents on their own enough? Honestly I'd appreciate any help I can get because right now I'm totally stomped and there's little to no information online that is remotely helpful.