Question: is a irish twenty pound note legal tender

Answer: No, Banknotes have been issued for use specifically in Northern Ireland since 1929, and are denominated in pounds sterling. They are legal currency, but technically not legal tender anywhere (including Northern Ireland itself). However, the banknotes are still widely accepted as currency by larger merchants and institutions elsewhere in the United Kingdom. Issuing banks have been granted legal rights to issue currency, and back the notes with deposits at the Bank of England.