Captain Jean-Luc Picard of the Federation starship USS Stargazer encountered the Skellig at some point in the mid-24th century. He knew they raided anyone unable to defend themselves to get anything valuabe.
In 2355, the Skellig came across the Rhitorri colony and started attacking it. The shields defended the Rhitorri city. Three days later, five Skellig raiders attacked the Starfleet cargo shuttle Nadir when it was making an emergency landing. Captain Picard, who had been aboard the shuttle, joined forces with the Rhitorri after they retrieved him.
When the city's shields fell, the Rhitorri expelled radioactive vapor from their towers, which prevented the large Skellig ships from beaming down troops. The Skellig wanted to capture the radiation-resistant Rhitorri to use as slave labor for radioactive environments. Skellig troops landed with small craft and attacked the Rhitorri in the underground. Picard fought them back before he lost consciousness due to succumbing to the radiation.
When the USS Wyoming arrived to respond to Nadir's distress call, David Capshaw led an away team helping repel the raiders. The Wyoming fought the Skellig ships, and the Skellig retreated. Picard was recovered and cured. Capshaw told him the ship would remain at the planet to protect the Rhitorri from further Skellig raids. (STA - Tales from the Captain's Table short story: "Darkness")