Having a medical card itself doesn't entitle you to the Back to School Clothing & Footwear scheme - entitlement depends on your income and income source.
You must be getting certain social welfare payments, or certain training, employment schemes or adult education payments, and your income must be below the limits for your family size.
The income limits, and further details on qualifying payments can be found on the SW website here -
If your only income is from employment and you have no SW payment (apart from child benefit) then you don't qualify.
If you are on FIS, that is a qualifying payment.
Edit: I've just seen your other thread, with your income details. As you are not on FIS (and your income appears to be over the income limit for FIS for 1 child - €506), you are not eligible for the Back to School Scheme.