Since you are in Ireland, brand names will vary. I would keep a vitamin /electrolyte tonic or powder to add to water or to give a few drops of with a very weak, sick, or dehydrated chicken. Amprollium is good for treating coccidiosis, and you may find it as Harkers Coxoid or Amprol where you live.
Antiseptic, such as chlorhexidene or povidone-iodine are good to have, as well as an antibiotic or antiseptic ointment which you may or may not be able to get locally. Coconut oil has several uses on skin conditions or for an impacted crop. Epsom salts may be helpful, added to warm water for a soak, in cases of scaly leg mites, prolapsed vent, or other reasons.
Antibiotics in general are controversial, and many people do not want to use them in their egg laying chickens. But if you need one in an emergency, amoxicillin can be used to treat wound infection or in reproductive disorders or infections.