A Goa'uld named Diancecht brought a group of people of Celt descent from Earth to the planet Herilia. In a bloody rebellion, the Herilians overthrew his regime hundreds of years ago, while his resources were spread thin because he was at war with another System Lord. Diancecht eventually returned to Herilia,
but the Herilians prevented him from conquering them again. Instead, he ruled in secret, with no one knowing he was in charge. He did this as an experiment to see what would happen if the Herilians would be more useful to him if they didn't know he was in charge. (RPG: "The Herilian Agenda")
This experiment on the Herilian society was not beneficial until the Herilians developed a technology involving Human bio-feedback, which he saw had combat applications. Another Goa'uld however, Arawn, was marooned on the planet and tried to claim it as his own. Diancecht secretly guided the Herilians against Arawn, which resulted in the marooned Goa'uld's capture. This allowed Diancecht to experiment on him with the bio-feedback device, as he was posing as a top-level bureaucrat on Herilia. (RPG: "The Herilian Agenda")
A member of the Stargate Command, Colonel Eriksen returned to Earth with suspicions about the Herilian people. Eriksen's team, as well as a group of secret agents, was tasked with finding out what the Herilians were hiding. If their mission went well and did not cause a diplomatic incident, relations could be formed between Earth and Herilia. (RPG: "The Herilian Agenda")
Much of the culture of the Herilian people is similar to their Celtic ancestors. Their technology is equivalent to that of England in the 1960's. Examples of Herilian names are: Alcidan, Allach, Allbhal, Arwaniad, Asuagan, Athan, Athlach, Bemnoc, Bessa, Bonan, Brell, Calla, Cenaid Cobarmach, Conana, Cun, Daimall, Dioride, Dullidan, Ebinnain, Ecain, Echarba, Elian, Etarmair, Faig, Geth, Iamall, Ianan, Kelochta, Lionn, Loarsagran, Lulboann, Maenire, Margran, Memain, Midhna, Molbavod, Mongil, Muaran, Nedar, Odlan, Olpar, Olra, Sassterna, Scona, Slirine, Tiann, Tuaine, Tunvodall.