If you've been on one parent family payment for a minimum of 12 months, then you can keep the medical card for 3 years after you start full time work.
Your One-parent family payment is means-tested so you will lose some of it, here's how it's assessed (taken from SW website).
The first €146.50 of your weekly earnings is not taken into account. This means that you can earn up to €146.50 per week and qualify for the full One-Parent Family Payment. (Social insurance contributions, health contributions, superannuation contributions and trade union subscriptions will not be taken into account in the assessment of earnings).
Half the remainder of your earnings up to €425 per week is assessed as means. If you earn between €146.50 and €425 per week you may qualify for a reduced payment.
You can apply for FIS, your wages and your one-parent family payment will be counted as income.
Good luck!