Immediately after take-off from Dublin, on a scheduled flight to Bristol, a bang was heard in the cabin area, which was followed by a blast of hot air, and what appeared to be smoke, entering into the cabin in the area of Row 10. The Cabin Staff Supervisor (CSS) alerted the Flight Crew. The Captain declared an emergency and made an immediate return to Dublin Airport. After landing safely, he ordered an emergency evacuation of the aircraft. Inspection of the aircraft found that a duct joint in the pressurisation system had failed. This allowed heated engine air to enter the cabin. The Investigation has made a total of four Safety Recommendations.