Plastic Paddy is a slur used to describe Irish in a derogatory way. Term used by the Irish describing those who grew up in the a foreign country (specifically the UK) and still identify as being Irish.
The word Plastic Paddy is a racial slur that has been used historically to degrade and dehumanize irish. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to irish. If you are unsure about whether or not it is appropriate to use the word Plastic Paddy, it is best to err on the side of caution and avoid using it in any situation.