I think Brian's point is that there would be no employment costs because the employees are already there.
Aer Lingus have a huge number of departures from Dublin before 7:30am, but after that things get a bit quieter, with only a few arrivals (places where they overnight like Heathrow or very short hops like Manchester where the return takes next to no time) coming in before 9:00am. But because of the shift patterns, they have baggage handlers employed there anyway.
Personally, I don't know enough about the baggage handling side of the industry to know whether they have other duties to perform at this time.
One thing I do know is that acountants would insist on assigning costs to it in a way that made it look less profitable, even though it would increase profits elsewhere in the business.
Whether it's doable, I don't know. If you look at the airlines coming in then (Aer Arann, Flybe, Air France, BMI), they have operations throughout the day and Aer Lingus couldn't just tender for the bits at those hours. There are a few US-based transatlantic carriers (American, Delta) who arrive then as well.