Bronte,
No, it's not related to medical cards, although the person getting the cert may have a medical card.
The SW medical cert is needed to apply for and receive SW illness benefit (e.g. those sick from work who have paid the appropriate PRSI). The GP issues the first medical cert (with the application form attached) and then issues cert(s) each week until the person is fit to return to work again.
SW pay a fee to the GP for issuing the cert, but as mentioned, the GP can and does charge the patient as well.