Hi Sir or Madam,
Benadryl is the drug we usually recommend for allergic reactions and the dosage is 1-2mg per pound every 8 hours. . Claritin and zyrtec have been used in dogs. The following site goes over antihistimines that are used and does give dosage information. I would consult your own vet before giving any medication to your dog besides benadryl.
Remember that many of these drugs have a different formula such as claratin-D. These are not safe for dogs and need to be avoided. You can use ice on the area to help and you should only use for about 5 minutes on and then 5 minutes off to provide some relief. Fire ants not only bite, but sting as well. Baking soda may help on the site as well. I hope this information is helpful to you.
Allegra or generic Fexofenadine can be used in dogs for allergies. The dose that is used is 2 to 5 mg/kg every 24 hrs. You should start with the lowest dosage first.
Hope this helps.