Fact is there is only a small population in Ireland to start with. When you cut to the chase many punters will happily fly into a hub like LHR or ATL in order to save a few quid on their ticket prices regardless of the hassle they will endure rather than go direct with AL. Many other potential fliers are put off by the flight duration.
The UK carriers have a much larger population base to rely on in the first place. This larger population will give them more people willing to pay business or first prices which is where the airline will generate most of its profits on the route.