During a scheduled passenger flight between Liverpool (EGGP) and EIDW, while in the cruise at FL180, the Cabin Altitude Warning Horn sounded. The Flight Crew identified that both air-conditioning pack switches were in the OFF position. The packs were then selected to AUTO. Subsequently, the crew doubted the operation of the pressurization system and an emergency descent was carried out. The aircraft entered a hold at 6,000 ft to allow the Flight Crew to review and confirm their understanding of events and the status of the aircraft. The aircraft landed without further incident. No injuries were reported.