Re: What happens if they refuse?
Actually, IMHO, the Commission can fine Ireland for not complying with EU rules. I don't believe it could fine Apple directly.
Apple applied to the court to block the Commission ruling because of course it's Apple that have to shell out the 13bn, not the Irish government.
What could really scare Apple is:
"Finally, all Commission decisions are subject to scrutiny by EU courts. If a Member State decides to appeal a Commission decision, it must still recover the illegal state aid but could, for example, place the recovered amount in an escrow account pending the outcome of the EU court procedures." (see http://europa.eu/rapid/press-release_IP-16-2923_en.htm).