Author and historian Tim Pat Coogan has told the Special Criminal Court in the Republic that the IRA and the INLA have "little time for each other".
Mr Coogan said that at times the two organisations would be "daggers drawn" and despite the ceasefire there were still personal animosities between them.
The author of a definitive history of the IRA said that the paramilitary world was "dangerous and conspiratorial".
He was giving evidence yesterday as an independent expert in the trial of three men who have denied membership of the INLA last year.
The three men were arrested in February last year by gardai investigating a plot to kidnap a Cork businessman.
Edward McGarrigle (43), of Melmount Gardens, Strabane, Co Tyrone, Neil Myles (54), of no fixed abode, and John McCrossan (47), of Ballycolman Estate, Strabane, Co Tyrone, have pleaded not guilty to membership to the INLA on February 22 last year.
It is the prosecution's case that the four men were involved in a plot to commit a crime at the home of a Cork businessman.
Mr Coogan told McGarrigle's counsel Brendan Nix that people in paramilitary organisations would be aware of "the players" in other organisations.
He said that they would have extensive knowledge of each others movements and they would know who the "players" were and their families.