A prison officer sent "evil and sadistic" anonymous letters to the son of murdered Brian Stack, pretending to be an IRA member and telling him his father deserved to suffer.
John Cooney (57) sent the sick mail as part of a poison pen campaign against members of the prison service after he became frustrated by promotion prospects.
Austin Stack, now a prison governor, was among 13 people in the service who were targeted over several years by Cooney, a court heard.
Mr Stack's father Brian, who was chief prison officer at Portlaoise Jail, was shot by the IRA in 1983, suffered brain damage, and died 18 months later.
Mr Stack said he felt "re-traumatised" by the three abusive letters Cooney sent.
Separately, a female prison worker was sent upsetting mail while suffering from a serious illness, while another officer was sent an obscene 40th birthday card.
Judge Cormac Dunne said Cooney had shown "excessive cruelty" and jailed him for a year, with another eight months suspended.
Cooney was released on bail shortly after, pending an appeal of the sentence.
Most of the content of the letters was not read out to the court as the judge deemed it too "vile and depraved".
Cooney, of Colthurst Road, Huntington Glen, Lucan, pleaded guilty to nine charges of sending indecent, obscene, offensive or menacing letters and five of harassment.
The offences happened between 2011 and 2015.
Mr Stack told Dublin District Court he began getting letters around the time gardaí began making headway in the investigation into his father's murder.
The first letter arrived to his workplace in Wheatfield prison. The others followed months later.
The author pretended to be from the IRA.
The letters stated that Mr Stack's father "deserved to be in pain" for the 18 months before his death.
"It was vile stuff, like my father deserved to linger, he deserved what he got, he was a bad man," Mr Stack told the judge.
This was "traumatic" for Mr Stack, who had been 14 at the time of the killing.
"I didn't like thinking about those 18 months when my father was like that," he said.
Defence barrister Paul Finnegan said Cooney accepted his behaviour was "an utterly unacceptable, unwarranted and inexcusable trespass on the lives of these people who were going about their work".