Before he came to America, MacCann was a soldier in the British Army against the Germans in 1917. He was stationed in Passchendaele, and saw his friends getting killed in the trenches, and decided that the military life was not for him. So he deserted, and fled to New York City where he became a mainstay of the Irish expat society.
He befriended many of the new immigrants, including a young man named Cassidy, who easily beat him at arm wrestling. MacCann introduced Cassidy to his fellow countrymen, and they made their home at McSorley's Old Ale House. There MacCann held court with his countrymen in their new home.
Interestingly enough, MacCann knew that Cassidy was a vampire before Cassidy did. Being relatively unread and uneducated, and without the dearth of vampire fiction that we have today, Cassidy had never considered exactly what that old swamp-hag's bite had done to him.
But MacCann had read Dracula, and knew exactly what had happened to him. He lent the book to Cassidy, which explained a great deal.
Eventually Cassidy left McSorley's, MacCann and his friends, afraid of having to watch them grow old and die. But they met again in 1975, just by chance. MacCann was older and walked with a cane, and Cassidy was unchanged, but they spent hours catching up. Before they parted for the last time, MacCann urged Cassidy to "Make the most of it." They never met again.