According to a biography of Brian, he once witnessed a raid on a Dal gCáis fort by the Vikings of Limerick. The fort was located on the banks of the River Shannon, allowing the Vikings to sail up the river from Limerick to attack it. According to the story, Brian, Mathgamain, and another older brother were on a hill or high ground near the fort tending to a herd of cattle. While they were there, they saw the raid from the mountainside after hearing screams and seeing smoke in the sky. They rushed down to the town, only to find the Vikings had already left. The settlement was burnt and looted. Brian's mother was killed, as were several of Brian's brothers who were defending the town, along with many of the townspeople. This event horrified Brian and had a lasting impact on him.