I think the obsession with direct US flights is primarily related to the impact it can have on the local economy rather than because it is of any benefit to the airport. Why did the company I work for open its first office on the European mainland in The Netherlands? Because there was a direct, frequent air connection from Cork. For business users, ease of connectivity is an important consideration when deciding on location.
I agree about Manchester. Indeed, with BMIBaby withdrawing from the East Midlands route, it's questionable what their value to Cork Airport is. They replaced an adequate Air Wales service to Cardiff with an adequate Air Wales service to Cardiff. They've forced BA off the Manchester route. They threaten the viability of Aer Arann's route to Birmingham. The only obvious benefit is the route to Durham Tees Valley and with Jet2 starting flights to Newcastle, it'll be interesting to see if they stick on that route.