The cocaine-addicted and paranoid crime boss, played by Aidan Gillen, was gunned down by henchman Darren in a lethal double cross planned by "lieutenant" Nidge, who has plans for John Boy's empire.
The shocking scene, which saw Darren enter a pub and shoot John Boy twice in the head was reminiscent of real-life gang boss Eamon Dunne's murder in a pub in Cabra in April 2010.
The huge ratings for last Sunday's episode, come after 511,000 viewers tuned in the previous week which saw Nidge target rival gang leader Fran's wife Linda in a pipebomb attack on their home. The gangland series has been a ratings hit for RTE and had an audience share of 41pc on Sunday night.
The last episode of the current series will be aired on Sunday.
It is expected to show John Boy's funeral -- and it's believed that close to a million viewers will tune in.
The drama, which also stars Robert Sheehan as Darren and Tom Vaughan Lawlor as Nidge, has already been sold to networks in Scotland and Australia and there are growing hopes the series will be picked up internationally as a ten-part series.
And RTE may commission a new series.