Hi Lori,
It's lovely to hear that you are with us for the Christmas and New Year!
The journey from Dublin to Cobh takes about 3 hours and uses the M7 motorway for most of the journey so depending on what time you land and leave Dublin will dictate when and where to buy your food to at least cover the Christmas Eve and Christmas Day.
It sounds like you are staying in self-catering rather than a hotel. Hotels usually have food for their guests if they are open. There are several large supermarket outlets that will indeed usually close by 6 pm on the 24th. As you will be leaving Dublin along the N7, there is a possibility of some open stores around the area including
Citywest shopping centre and some local
Supervalu stores as well as in Swords close to the airport.
Have you asked your accommodation provider if there is a way to get food delivered maybe ahead of your arrival?