 Synge's paternal grandfather, also named John Synge, was an evangelical Christian involved in the movement that became the Plymouth Brethren and his maternal grandfather, Robert Traill, had been a Church of Ireland rector in Schull, County Cork, who died in 1847 during the Great Irish Famine.His great, great grandfather was the archdeacon of killala 

Synge's father contracted smallpox and died in 1872 at the age of 49.He was buried on his son's first birthday.Synge's mother moved the family to the house next door to her mother's house in Rathgar, County Dublin.