OK well taking each example as they come and using the calculator on taxcalc.ie:
1. Joint assessment with your income first I get a take-home pay of €56,730 for 2015. I think this should be €56,930 as the tax band for a married couple with 2 incomes is €67,600 and not €66,600 as per the calculator
2. Joint assessment with your husband's income first I get a take-home pay of €58,380 for 2015. This is actually incorrect as the self-employed spouse has been granted the PAYE credit in error. The correct figure is again €56,930
3. Separate assessment (effectively taxed as 2 single people)
You will come out with €19,755
Husband comes out with €35,415
Household income total of €55,170
So the difference between the joint income of €56,930 and the separate income of €55,170 is €1,760 which is how much better off you would be by being taxed jointly.
This would agree with the logic of you transferring the remainder of your 20% band to your husband and this band now being taxed at 20% instead of 40%:
€33,800 - €25,000 = €8,800 @ 20% saving = €1,760
My original calc of €1,848 is incorrect as I was using a 21% difference instead of a 20% difference (€8,800 @ 21% saving = €1,848)
Lot of figures there but that should be OK for you