Police in Northern Ireland blamed the Irish Republican Army for a double bombing that killed a Roman Catholic man, injured 15 people and damaged 50 houses in a mainly Protestant housing project in Londonderry. Six children were among the injured. The outlawed IRA denied responsibility, saying it would not bomb Catholic homes. Two time bombs, left in small bags on doorsteps--one of them at the home of the man who was killed--exploded within two minutes of each other, police said. A third bomb, also in a bag, was found on a road about 100 yards from the housing project, but it failed to go off and was disarmed. The attackers apparently tossed this bomb from a car as they fled the housing project, police said.