Hi
Just because you paid in €3600 does not mean you can cash your plan in for €3600 - the cost of your cover is taken from the plan value. chances are you plan will be worth a few hundred.
The cheapest cover you can get is called joint life, decreasing term cover.
Just say your mortgage is for €200,000 over 30 years. Your initial level of cover will be 200,000 which will reduce every month approximately in line with your mortgage. If either of you die the policy will pay out and end, and the money will be used to pay off your mortgage. The premium does not change - it remains the same for the full 30 years.
The premium is determined by the amount of cover, time scale, your age, health, gender and smoking status.
You were obviously sold a much more expensive form of cover. If you believe you were mis-sold the policy you should write a letter of complaint to BOI. If they don't put things right you can make a complaint to the financial services ombudsman.
PS I took out a single life policy which only cost €13 a month. I later replaced it with one which also covered specified illnesses and it costs €25 a month.