3kids - Illness Benefit is not means tested but Disability Allowance is. It still seems likely that he is getting Illness Benefit as you don't have to send in medical certificates for Disability Allowance.
Assuming that he is getting Illness Benefit the following would apply if you got a job for 20 hours a week with weekly earnings of €220.
Option 1 - remain on your husband's claim as a qualified adult (no FIS paid). His Illness Benefit would be made up as follows:
Personal Rate............................................. €197.80
Reduced Increase for a Qualified Adult............. € 57.50
Qualified Child (x3)....................................... € 72.00
Total....................................................... €327.30
Total weekly income would be €327.30 + €220 = €547.30
Option 2 - apply for FIS and your husband would lose the payment for you and get half-rate for the children. His Illness Benefit would be made up as follows:
Personal Rate............................................€197.80
Half-rate for 3 qualified children.....................€ 36.00
Total.......................................................€233.80
Total weekly income before FIS would be €233.80 + €220 = €453.80
Family Income Supplement calculation
Income limit for 3 children is €655. €655 - €453.80 = €201.20. FIS payable would be 60% of this which equals €120.72 (rounded up to €121)
Total weekly income after FIS would be €453.80 + €121 = €574.80
You should contact your Community Welfare Officer to see how your Mortgage Interest Supplement will be affected. FIS is not taken into account for this.
Illness Benefit is taxable after the first 6 weeks of each tax year but the Child Payments are not taxable.
I've assumed no tax or PRSI when doing the calculations.
Hope this helps as it gives you an idea of how to do the calculations.