Since you have an offset mortgage leaving any excess on the related current account means you will be saving interest on that amount of savings at 3.1%
Compare this to the interest rate you would receive from being pacing it in deposit, e.g. Rabo Direct are offering 2.4% (3% less DIRT), Northern Rock might be slightly better have a look at
http://www.askaboutmoney.com/clubman/BestBuys.htm
So in the short term that looks best
3% return on your money may not be great but some people would like the safety of the money being availible at any moment
If you would like to reduce your PPR I would recommend leaving the excess in the mortgage related current account (reducing intwerest due each month)and when it reaches a level that you are happy to have as a cash reserve (in case of a rainy day) everything after that (usually once a year) pay into PPR mortgage account
From a tax point of view the last loan you want to pay off is the investment property loan, this attracts tax relief (up to 47%)
You own home loan (unless your area first time buyer) gives you tax relief after the first €5080 of interest paid (and still it is at 20%) so youwill get no tax relief on that loan for anything above €163,871
The next issue you should address is future cashflow needs and your own investment risk concerns
If you want to invest the money there are loads of options but these depend in your personal attitude to risk
Whether it me property, equities, funds, etc
If you post more info I am sure people will help guide you in that area
stuart@buyingtolet.ie