The only man convicted in connection with the Omagh bomb is being shunned by fellow republican prisoners because of claims he has cooperated with gardai.
Colm Murphy, who has been ostracised by both Real IRA and Continuity IRA inmates at Portlaoise prison, is being forced to eat meals in his cell and exercise alone.
Sources say Murphy, serving a 14-year sentence for his role the 1998 Omagh bombing, has been ostracised because of claims in the book Black Operations: The Secret War Against the Real IRA that he had surprised garda interrogators with his "frankness and honesty" during interviews in the wake of the blast.
A former Portlaoise prisoner said dissident republicans were infuriated by the suggestion that the Co Armagh man had spoken "freely" to detectives.
The source also said Murphy is now speaking with just two of his fellow inmates, an INLA man and Patrick 'Dutchy' Holland, the Dublin criminal cleared of the 1996 murder of journalist Veronica Guerin.
It is understood that the relationship has raised further objections from RIRA prisoners who do not want Holland visiting their landing.
RIRA bosses Michael McKevitt and Liam Campbell, both locked in a bitter feud, as well as CIRA chiefs, are said to be furious with Murphy over the claims made in the new book.
"This book has caused tremendous trouble and it is only a matter of time until there is a row in the prison between the hardliners," the source said.
Murphy, a longtime CIRA member, is also said to have angered dissident bosses outside prison because of his efforts to have his organisation call off its own terror campaign.
"There is awful tension in the prison as the leadership of the CIRA on the outside are opposed to calling a ceasefire," the source said.
Murphy is one of five prisoners being sued by the relatives of the Omagh victims over their alleged role in the explosion that claimed the lives of 29 people and two unborn babies.
During his trial, the prosecution argued that the father-of-four had lent his mobile phone and a second phone belonging to an innocent party to the people responsible for planting the bomb.
He is the latest hardline republican to clash with fellow inmates in Portlaoise Prison.
In October 2002, the prison authorities were forced to move Dundalk man Liam Campbell from E4 Wing to D-Block after he was threatened by his former Real IRA associates at the top security jail.
He remains on a wing normally occupied by non-subversive prisoners.
He is facing a retrial after the Court of Criminal Appeal last month overturned his conviction and five-year sentence for membership of the RIRA.