They say this is the guy for whom the River Shannon in Ireland is named, but that's not the cool part of his story. Nor is his refusal to allow any woman to set foot on Scattery Island, where he established the last monastery at which he lived. He established several, and lived at even more, but none of those are the points I want to talk about either.
The cool thing is that when he first moved to Scattery Island, he found the residents living in terror of the Cathach, a horrible sea serpent. It was also called a peist, a type of gold-guarding monster found in the lakes of Ireland. Senan climbed on top of the highest hill on the island and spotted the monster. He marched straight to it and ordered it in the name of the Trinity to depart and not return. It did, of course, diving to the bottom of Doolough Lake at the foot of Mount Callan.