He is not the son of Tony Grealish - who was born in London.
This from Wiki (I know, I know)
Grealish was born in Solihull in the West Midlands to an English mother and a Birmingham-born father. He qualifies for the Republic of Ireland through his maternal grandfather from County Dublin, his paternal grandfather from Gort, County Galway, and his paternal grandmother from County Kerry.[3][4][5]
Though born in England, Grealish has so far opted to represent the Republic of Ireland at international level. Grealish was called up to an England U15 training camp but was sent home after falling ill; since then he has represented the Republic of Ireland since first being called up at the age of 14 and has won caps from under-15 to under-21 level.
England are known to have been pursuing him, even naming him in their U17 squad in 2011 at the age of 15 – an invitation he declined.