Harp is a slur used to describe Irish in a derogatory way. Perhaps derived from the fact that Harp Ale is brewed in Ireland or that the Harp is popular instrument amongst the Irish. The harp is also the symbol of Ireland, a green flag with a harp is displayed by a lot of Irish people.
The word Harp 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 Harp, it is best to err on the side of caution and avoid using it in any situation.