Alan Warner used the term mortal to describe someone who is
drunk in his book
Morvern Callar. Here is an example:
"We were all is hysterics at that then Panatine materialized with the whiskey.... a good lot more ceilidhing went on and we were all mortal as newts."
In other words, if you are mortal and you drink you get drunk.
So his characters ask each other, "Are you mortal?" - or are you drunk? I guess if you were immortal and you drank you would not get drunk.
Warner is a Scotsman, so this may simply be common slang. However, he is also fond of neologisms so I am not sure.
By the way, if you are fond of Irvine Welsh, you would probably like Alan Warner too.