It's so lovely that you are thinking of celebrating your 20th anniversary here with us on the island! Galway
is a great place to explore the Wild Atlantic Way
from, and there are so many things to see and do in the area. What do you have in mind for your trip so far?
Travelling back from Galway on the day of your flight is not a crazy idea at all. :) If you are flying at 11:30 am from Dublin, you could take a bus from Galway directly to the airport early in the morning. Dublin Airport
recommends to check with your airline to see how long before your flight to arrive at the airport, but advise that it should be no less than 90 minutes before departure.
Galway to Dublin Express stops at the Airport, and this direct route will take you there within 3 hours. This service runs every hour, 24 hours a day, so you could leave as early as you'd like. Take a look at the Transport for Ireland
page for more direct bus routes. Would this be an option?