Answer: the The essence of Irish music is the spirit of the music.
It involves the lyrical themes, the types of instruments used, the rhythm and the way it is structured. It may involve the dances that typically go along with it.
"Real" Irish music may be more difficult to define. It may refer to music composed by Irish musicians in Ireland. It may mean the traditional music that people associate with Ireland. There is also an Irish Reel-- different spelling-- a type of dance with specific structure.
Explanation: There is a long "debate" about the Irish music associated with the American celebration of St. Patrick's day- "McNamara's Band," "Toura Loura Loura," "Danny Boy," etc. and how it is much different from music actually played in Ireland: ballads, folk songs, jigs and reels. I hope you are able to listen to some examples and judge for yourself.