McGann was general of the Minutemen in 2240. He led the Minutemen after the beginning of their collapse in 2247, and was stationed at the Castle when it was taken over by mirelurks. He mistook the mirelurk queen for a behemoth leading a group of super mutants. Fearing that the mutants would make use of the fort's artillery, he locked down the armory, setting up traps and activating the tunnels' security measures. Finding out that he was trapped in the basement, McGann passed the time drinking the ancient Amontillado wines stored there, until he died of starvation/dehydration.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
General McGann is seen with two bottles of amontillado. Nearby, there is a skeleton chained to the inside of the wall opposite him. This is a reference to Edgar Allen Poe's The Cask of Amontillado, in which the antagonist is enticed into a cellar to taste the expensive wine, but is instead trapped behind a wall where he is left to die. Interestingly, the inspiration for Poe's The Cask of Amontillado, may have come from his time on Castle Island (South Boston).
End of information based on unverified behind the scenes information.