On landing at London Heathrow earlier in the morning, a noise from the forward hold area was reported by the flight crew. An inspection by ground crew found a blow out panel open. Finding nothing amiss, the panel was reset and the aircraft released for the return flight to Dublin. Whilst the aircraft was climbing through FL266, the flight crew received an ECAM (Electronic Centralised Aircraft Monitoring) warning that the cabin altitude was excessive and a subsequent warning of low cabin differential pressure. An emergency descent was initiated and executed without further incident at Cardiff Airport.