One of the Tuatha Dé Danann, Airmed was the daughter of Dian-cécht, the healer god. Airmed was great healer and herbalist, better than even her father.
When the king of the Tuatha Dé Danann, Nuada, lost his arm in battle, Dian-cécht attempted to heal it, but the best he could do was to make a silver prosthesis. Since the king of the Tuatha Dé Danann was required to be 'whole', Nuada could not rule with his silver arm*, and was replaced by Bres. Bres turned out to be a bad leader, and was overthrown. Luckily, while Bres was making a mess of things, Airmed and Miach (her brother, also a healer), healed Nuada's arm completely. Dian-cécht killed Miach in a jealous rage** (Dian-cécht was the head doctor, and didn't like his children showing him up). On Miach's grave there grew 365 healing herbs, which Airmed collected and sorted by their healing properties. This angered Dian-cécht, who mixed up the herbs, so no-one could tell which did what -- except for Airmed. Thus Airmed is the Goddess of herbal healing; is you have a question about herbs, you go to her.
* In some versions, the silver arm was sufficient, and Nuada took the throne back from Bres as soon as it was in place. But even in this version Dian-cécht kills Miach after he and Airmed heal the arm.
** Dian-cécht claims that he didn't mind them healing the arm, but the method they used was unacceptable. I don't know what this method was.