Fairytale
Just a few questions so that you question can be answered.
1. Is your monthly salary of €850 gross (before Tax, PRSI etc.) or net (after Tax, PRSI, etc)? One-Parent family is worked out on gross income while Family Income Supplement (FIS) is calculated on your net income.
2. Do you get maintenance from the other parent of your child? This may also be taken into account for both payments. Although maintenance payments by way of a court order are not assessable for FIS.
Assuming the figure of €850 pm is a net figure and that you don't get maintenance for your son - this is what you can hope to get assuming you satisfy the other conditions.
Your weekly income is €196.15 (850 x 12 / 52)
First of all your One-Parent Family Payment (OFP) is calculated. €196.15 - €146.50 = €49.65. This is divided by two to give weekly means of €24.82. This would give you a weekly OFP of €190.30
The FIS calculation is done by adding your income (€196.15) and OFP €190.30 giving €381.45. The amount of FIS that you would get is determined by taking this total from the family income limit for 1 child (€480) and getting 60% of the difference.
€480.00 minus €381.45 equals €98.55. 60% of €98.55 equals €59.13 which would be rounded up to €60. So your total weekly income would be €441.45.
I know I'm making several assumptions ( so don't bite me) but I just wanted to show how the calculations are done. You should apply for the OFP now and not wait until July.
Here are some useful links
http://www.welfare.ie/publications/sw82.html#5
http://www.welfare.ie/publications/sw19/sw19_sect3.html#3.7
http://www.welfare.ie/publications/sw19/sw19_sect7.html#7.1
Hope this helps