Stick the money in a regular monthly savings account. Anglo are currently paying 7.3% next best is BOI at around 4.75%. Your tracker mortgage is probadly around 1.25% over ECB so the interest is likely to be 2.5% currently. So its a case of do you want 7.3% (less 1/4 in dirt) or 2.5% (less probadly 1/2 of 2.5% in additional tax on the rental income). Down the line you can use the lump sum saved to either pay of some of the mortgage or as a rainy day fund and believe me there are a few wet days ahead (for everone).