Isaac was a part of the family that ran the Garrahan Mining Company. Unlike the other members of the family, he didn't devote a great deal of his time to the company, instead preferring to spend time with his pet sloths. He went so far as to call himself their savior and master.
This was not well accepted by Isaac's brother, Bill Garrahan, who tired of the sloths sneaking into his room and scaring him, subsequently threatening to kill any sloth that escaped after that. Following the Great War, the sloths escaped and mutated into mega sloths.
Isaac Garrahan is mentioned only in Fallout 76.