During take-off, the Tail Skid Assembly shoe on the underside of the aft fuselage came in contact with the runway surface. The aircraft continued on its departure, and levelled off at Flight Level (FL)120, where the Flight Crew then completed the ‘Tailstrike on Takeoff’ checklist. While completing this checklist the aircraft was depressurised. After communication with the Cabin Crew, the passenger oxygen masks were deployed manually. The Flight Crew declared an Emergency and returned to Dublin Airport where the flight landed without further incident.