If you've taken the job then definitley consider moving there. I spent most of the last 5 years travelling up to Dublin, staying in hotels during the week, then back to Cork at the weekends. I had an apartment in Dublin for the last year but still travelled home most weekends. Aside from being expensive running 2 places it was very tiring, packing the bag on Sunday evening was especially miserable. Someone suggested looking at a B&B, this would be worth doing for Monday & Thursday which would make the daily commute a little bit better, however also have a look at a house share option in Limerick. It might work out the same as a B&B for two night a a week, or even the same as the fuel for commuting.
In the end though I think you're better off moving up there fulltime & make a proper go of it.