A Mass of Remembrance for the victims, who were killed and buried secretly during the Troubles, was held this afternoon at St Patrick's Cathedral in Armagh.
Cardinal Brady said anyone who could help locate the bodies should do so.
The Catholic Primate of All-Ireland said he hoped that those who have even the tiniest item of information would have the courage to pass it on and so help to ease the pain of the families who he said had suffered so much for so long.
The remains of five of those killed by the IRA and secretly buried have so far been found.
They are Eamon Molloy, Brian McKinney, John McClory, Jean McConville and Danny McIlhone.
The IRA admitted in 1999 that it murdered and buried nine people.
The four people, whose remains have so far not been discovered, are Seamus Wright, Kevin McKee, Columba McVeigh and Brendan Megraw.
Others who vanished in south Armagh during the Troubles include Gerry Evans and Charles Armstrong, thought to have been murdered by the IRA, and Seamus Ruddy, who disappeared in France and whose murder was admitted by the INLA.
The body of undercover soldier Captain Robert Nairac who was abducted and killed by the IRA in 1977 has not been found.
