At its simplest, Mortgage Interest Supplement is calculated using the following figures:
- household income (in your friend's case One Parent Family Payment and Maintenance)
- the equivalent SWA rate for the family size
- the amount of interest paid on the mortgage calculated as a weekly amount.
- a minimum contribution of €24 pw which your friend must pay herself.
MIS is calculated as follows:
Add all household income together and subtract the equivalent SWA rate.
The difference is the "income in excess"
Add the minimum contribution to the income in excess amount and subtract the total from the weekly interest portion of the mortgage.
From the information given, it sounds as if the income in excess plus the minimum contribution total is €4 greater than the weekly interest portion of the mortgage.
The amount of maintenance paid is disregarded when assessing entitlement to One Parent Family Payment - as it can't be disregarded twice, the maintenance is assessed in full for MIS.
Did your friend claim Jobseeker's Benefit (JB) when she became unemployed? She may have an entitlement to half-rate JB in addition to her OFP, depending on her PRSI contributions and availability for work. If she qualified, it would push her further over the limit for MIS, but it would be extra income.