I would just play it by ear. After your first 2 or 3 days of driving and seeing some sites you should have a good idea of whether you've bitten off more than you can chew comfortably or you may find you're itinerary is doable. I would sit down and list the places you'd like to see in a priority order (not an itinerary order but a value order starting with "must see's" down to "could live without") and then when you commence your trip and it begins to feel a little rushed, just skip a convenient "could live without" place and move on.
Just make sure you hit your "must sees" and consider the rest the icing on the cake. Ireland is so beautiful pretty much anywhere you go, you can't go wrong.
If you've never been to the southwest, then by all means you need to go explore it. But for me personally, having lived in Ireland for a couple of years and seen most of it (never stepped foot in the North though!) I would spend most of my time in Connemara and up in Mayo and Donegal. The southwest is beautiful but the northwest is a bit off the average tourist routes so much less traffic and has such uniquely lonely and spectacular landscapes.

Any direction though, you can't miss. (Although Wexford and the southeast, while it does have its own charms, does tend to pale a bit in comparison with the striking landscapes of those other areas.)
So basically, go with your plan in hand, and then just adjust as necessary.
(Although you probably couldn't do your little U.K. side trip on the spur of the moment so you'd have to decide that one way or the other beforehand. Other than that, just go and get lost on some obscure backroad in Co. Mayo. The best memories will most likely be just such unplanned events!)