Fergus, whose name means Man's Strength, was an Irish bishop who undertook missions in Scotland. He founded several churches. I have only read two interesting things about him.
First, he attended a council in Rome in 721. That council condemned sorcery and irregular marriages. That may not seem like a big deal, but previous councils had been debating things like whether Jesus was begotten or made and whether a monk's tonsure should be at the back of his head or side to side across the top, so condemning sorcery seems like a step forward to me. Of course, disclaiming sorcery would be an even bigger step forward, but I'll settle for baby steps.
Second, the abbot of Scone acquired his head during the reign of James IV (Fergus had been dead for centuries) and built an elaborate shrine for it. Aberdeen has one of his arms -- I don't know who has the other.
I don't have any deep thoughts about Fergus, but he seems like a good guy and I don't write about the Celts as often as I'd like.