Er, possibly because it's a long way?
If the aircraft was able to fly in a straight line, it would have to travel 3125 miles. If it took the full 8.5 hours for the journey, that would give it an average speed of 368 mph (which isn't exactly slow!).
However, the aircraft has to follow international air corridors, which add to the distance. (The flight path takes the aircraft into Canadian airspace).
The 8.5 flight time includes the time 'pushing back' and taxiing at Knock plus the almost inevitable time stuck in a stack over the Atlantic, waiting to land in New York. So it's probably more likely that the aircraft is really covering, say 3600 miles in 7.5 hours. That would give it an average speed of 480 miles per hour.
If FlyGlobeSpan use their newly-acquired 767-300 aircraft, they can cruise at 530mph. But FlyGlobeSpan might have built enough time into their schedules to allow them to operate their slower 737 series aircraft (with cruising speeds of around 510 mph).
However, the flight from Knock to NYC is against the prevailing winds, so the actual speeds will be lower. (Additionally, aircraft can only achieve their cruising speed when they've climbed to their optimal cruising height. They travel slower at lower altitudes). So an average of 480 mph would seem reasonable.
Putting all of those facts together seems to make the sums add up to show that 8.5 hours is a reasonable time for the flight.
Chris