Hi KF,
The military exercise areas around Dublin are no big deal - many are "active" only at weekends, and even when active I've invariably been cleared through them without any hassle.
As others have said flight plans are mandatory for crossing an international FIR boundary, and you will need to file a GAR for your flight back to the UK 12 hours in advance.
Suggest you put a notice up at your airfield (guessing Sleap?) asking if any experienced ppl would like to come along for the ride in return for help with planning and the formalities. You could ask something similar on the "spare seats" thread on this forum.
As for routing, if it was me I'd route Sleap-Newcastle direct, asking for clearance through Dublin CTA at my desired crossing level , or descending to <2500' and going under it in the unlikely event clearance refused.