Can I have some Irish Sausages, please?' asked the Irishman, walking up to the counter.
The assistant looked at him and asked: 'Are you Irish?'
'If I had asked you for Italian sausage, would you ask me if I was Italian?' demanded the Irishman indignantly. 'Or, if I asked for German Bratwurst, would you ask me if I was German?'
Then, warming to his theme, he went on: 'Or if I asked you for a kosher hot dog, would you ask me if I was Jewish? Or, if I asked you for a taco, would you ask me if I was Mexican?
Would Ya? Would Ya?'
The assistant said: 'Well, no.'
Suitably encouraged by the success of his logic, the Irishman steps it up a gear 'And if I asked you for frog's legs, would you ask me if I was French?'
'What about Danish Bacon, would you ask me if I was Danish?'
'Well no, I probably wouldn't' conceded the assistant.
So, now bursting with righteous indignation, the Irishman says: 'Well, all right then, why did you ask me if I'm Irish just because I asked for Irish sausages?'
The assistant replied: 'Because you're in Homebase'
(Thanks to MR)