
The only evidence to suggest it is brandy is a painting by Landseer in 1831 - (the actual portrait by Sir Edwin Landseer, I could not find, so I have added this one).
Brandy would kill someone with hyperthermia.
The dog in the painting was called "Barry", who managed to rescue 40 people before being shot by the 41st person he was due to rescue, who thought Barry was a wolf.
In honour of Barry, the most handsome St. Bernard at the Great St Bernard Pass hospice is called "Barry".