I suppose a very simple calculation, would be to decide what income you would like to have, in the event of a spouse dying. Then calculate the benefits payable under their pension, work out shortfall and work out the lump sum that would pay that amount on current bank returns.
its crude, but is would give a guideline figure. remember, mortgage will be paid off, you will have a survivors pension (assuming spouse paying PRSI) you may have a spouss pension from his/her workplace, but again check the wording of that policy through his/her HR dept as some will stop paying pension should you remarry.
If taking death of wife/mother into account, do not forget to add on for housekepping expenses, childminding etc as no mater what you think or how liberated/metrosexual you are, womans work in home is usually undervalued for financial purposes.