An alternative is to put your excess funds into a deposit which earns at or close to the fixed rate, such as Anglo/BOSI which pay upwards of 4.5%. Obviously you have to pay DIRT, but at least the funds are accruing and capitalising interest.
When your fixed period on the mortgage expires, transfer the deposit to the mortgage.
Further point, depending on the size of mortgage, you will be earning TRS on the Interest on the portion of mortgage which you have not paid off, somewhat offsetting the DIRT paid on the deposit.
Final point, I think if you can get fixed at anywhere around the 4.25 - 4.50 level for > 2 years would be worthwile allowing for 2 25 pip hikes on the ECB repo rate to 3.5% by Xmas '06. This 4.5% being similar to the depo rate mentioned at the start above. I don't see value or point in fixing to 5 years as the premium is too high with most banks, and also quite inflexible for most people unless you plan to stay in the same house for 5 years plus.