You won't need a full lunch and dinner on both days especially if you have breakfast at the hotel which will be a 'full Irish'. The main Christmas meal in Ireland is Christmas lunch which when served at home is usually about 3pm and keeps most people satiated for the rest of the day maybe with the odd mince pie, piece of Christmas cake or chocolate later in the day.
On Christmas Eve most places should be open at lunchtime and also quite a few should be open for an early dinner. I would imagine that only hotel restaurants will be open on Christmas Day. I suggest that you book your main Christmas day meal, be it lunch or dinner at the hotel and stock up on a few snacks such as cheese, crackers, smoked salmon, mince pies etc to tide you over for the rest of the day. There is a Marks and Spencer in Grafton St and if they have a food department in that branch that would be a good place to get your supplies -they also do ready made sandwiches.
P.S You will be lucky if Christmas Lunch/Dinner only costs 100 Euros