The advice is based on their expectation that interest rates will rise to a rate greater than the current fixed rates on offer (approx 5% for 5-year fixed rate). The ECB has said that it intends to keep interest rates low for the short term (6-9 months). After that, they only way is up; it's only a matter of when, and my how much.
In addition, even if ECB keeps interest rates low, the banks need to make money and so are looking to raise rates for those on other than a tracker rate. This is evidenced by the recent increases by most of the Irish banks and many believe that these increases will be followed by more.
I have no advice as to what you should do but that's the reason that people are advising mortgage holders to fix.