Irish Nationwide sometimes put a clause into thier loan offers concerning thier fixed rates. This allows you to book your rate, which was quite common during the dizzy heights of the early 90's.
They are one of the few lenders that allow you to book a rate to avoid this particular event happening. Check your loan offer and if you used a broker check with them, who may be able to help. However, I am almost certain that there is very little you can do about it except move after the three year fixed rate.
I don't necessarily agree that INBS rate are the most competative on rates however how bad is the new rate compared to other lenders. What rate did it close at? Was the increase just INBS moving in line with the market (albiet slowly)
BTW - I have no connection with INBS - just trying to give balanced view.