The general formula for calculating mortgage interest supplement is as follows:

1. Calculate total weekly income - this can be SW payments, part-time earnings, assessed savings or other property.

2. Subtract the SWA rate for the family size from the total weekly income. This is the income in excess. As a guide, SWA rates and full-rate JA/JB rates of payment are the same (€204.30 for a single person)

3. Calculate the weekly interest on the mortgage.

4. Entitlement is calculated as - weekly interest on mortgage minus income in excess minus minimum contribution (€24).

If the amount reached at no 4 is positive, then that's the entitlement, if it's negative, there is no entitlement.

There may be other factors which affect entitlement - if the interest is considered very high, then entitlement (if any) may be based on a lower amount rather than the full interest paid, or if there's a non-dependant adult child residing, they are expected to contribute towards the mortgage as well.