Applying some of my new-found knowledge here..
Fixing makes things like switching provider or paying off some of your mortgage early very expensive. If you don't plan on doing either of those the only problem is that you may be stuck with a higher rate than the standard variable.
General consensus is that ECB rates are only going one way - up. Most seem to predict a raise of up 1% this year (going up in increments of .25% - starting perhaps as early as next month), with a further .5-1% next year. After that, it's anyone's guess. Have a google and do some of your own research on this.
Lots of banks seem to be pulling their fixed rate products or making them prohibatively expensive. For me, that seems to be evidence that fixing on a decent rate right now would be a good idea.
I'm in the same position as you and am very much considering fixing for 5 years.
FYI, I believe AIB has a better 5 year fixed rate, at 4.39%. If anyone knows of a better rate than that, please let me know.