I am a few years older than you, but when I had the oppertunity last year, I put the bonus into AVC. the reason being that some colleagues of mine are pumping all they can into AVC in their 50's, which is too late. If you put into AVC now, it has 25/30 years to grow and growth in some equity fund should be good over that period. teh entire €10K goes into AVC.
The alternative, is that if you want to put it into reducing mortgage, you must presumably pay income tax on it fist, which beings the sum available to about €5000, taking tax and PRSI into account. that would probably save you €25 per month, or €300, but would have the outstanding balance reduced by €5K.
I hope that this is helpful.