I have never had to wait an hour for a flight plan to be accepted, although I usually flie the night before anyway.
If you phone Shannon AIS on 00353 61471233, very often they will let you go in about 15 min, Unless you intend to transit Dublin, Cork or Shannon CTR or TMA.
You can file airborne quite legally at least 10 min before crossing the FIR.
If your really "bad" and your in a private airfield you can take off, call Dublin and ask them to open your flight plan, (the one you "should" have filed with Shannon), They will take your details.
I must tell you that I have NEVER done that myself but I know a few who have.
Also when crossing the Irish sea give D&D a call on 121.5 just to say hello, they will be glad to hear from you.
Have fun and stay safe
Tony