First things first. Make sure it is booked directly with Aer Lingus via either the website or via "travel fusion" otherwise it's a travel agent ticket and you won't be able to upgrade that easily.

Cash wise, don't use the website call and ask for a paid upgrade, agents may waive the 100 euro change fee.

For Avios, try a dummy booking with Avios and see if there is availability, if there is email [email protected] with the details requesting an upgrade. If there is no availability then it won't be confirmed and you are put on standby for business and it could be approved next day or maybe not until 2 days before flight or doesn't get approved at all.