The play was premiered in 1965 and was partially based on a 1959 incident, so it's set around the early 1960s. The fact that electricity is a novelty in the area reinforces this date - many rural areas of Ireland were not electrified until the 1960s or even the 1970s.
Is this film based on a novel?
No, it is based on a 1965 play of the same title by John B. Keane. It was partially inspired by the 1959 murder of "Moss" Moore in Reamore, County Kerry.