Any guarantee that they would use them on Irish routes?
Only for the required 3 years. There is an article in the FT online which says that BA could use the slots to fly elsewhere and that it is desperate to get its hands on more slots since the 3rd runway has been canned and LHR is at capacity.
I am wondering with these "remedies" if MOL hasn't shot himself in the foot. He never cared about LHR anyway as he had no intention of operating into there but would get lots of cash from selling/leasing the slots. Flybe wouldn't worry him in the least operating into Ireland since at the end of the 3 year period a "seat sale" would soon push them out of the country and hey presto its back to a monopoly and the good ole days are back.
With the growth in the Irish flying through the ME and connecting onwards with EK and EY I can't see BA keeping the Irish routes going after 3 years if they were successful in getting the slots. Also with the US carriers, CBP and hub connectivity through the east coast there would only be a few routes that people would find LHR worth flying through. Probably not enough to justify BA flying into Ireland for.