'There is no difference in beheading or shooting'
Columba McVeigh was abducted, murdered and secretly buried by the IRA in 1975.
The IRA admitted to killing him and claimed the 17-year-old confessed to being an informer - an allegation strongly denied by his family. He along with 16 others subsequently became known as the Disappeared.
The Independent Commission for the Location of Victims' Remains was established to help reunite the families with their loved ones.
Columba and seven others are still to be found.
His brother Oliver said: "What they did to Columba was evil.
"They have said where he is, but not exactly. Nothing is worth one drop of blood whether it is the IRA or Isis.
"The two are similar, but they are not the same.
"There is no difference between beheading someone and shooting them in the head in cold blood."
He added: "At least the IRA had a cause, they said they were oppressed and wanted the British out of Ireland. They just went about it the wrong way."
Bishop Dolan said the activities the IRA were engaged in were "not Catholic".
"He's right," said Oliver. "They are not even Christian. You have to be Christian first before you can be Catholic."
Asked if he thought Isis members would relinquish violence for peace in a similar way IRA members have, he responded: "They are too radical. No one will be able to influence them otherwise."