I found some shakes from Carb Options in Creamy Vanilla and Dark Chocolate. Both have about 3 net carbs

The dark chocolate drink is sickly sweet so is the creamy vanilla, but somehow actually like the creamy vanilla.

Also saw some LC croissants from Boots and chocolate covered peanuts, also from Boots, but quite high in polyols though.

Holland and Barrett close to the St.Stephen's Green also had some cake mixtures. You just have to add some water and one egg, stick it in the micro for a few minutes - and hey presto you have some cake. It looked quite tasty, but if you go with the 3 or 5 minute cake from the recipe section, you'll probably will get a healthier and tastier cake ;-)

Boots also has some bread mix and packets of soups (tomato and mushroom) which could be handy for work or if you are in the rush.