You don't have an automatic entitlement (persons whose sole income is SW do), but you may qualify on a means test.
The 2009 medical card income guidelines have a limit of €266.50 pw for a couple for a medical card or €400 for a GP visit card.
Your wife's wages are assessed after tax and PRSI (different to SW who don't exclude tax paid), and there are allowances for travel to work and rent/mortgage costs. The income limits would also be increased when the baby is born.
Best thing to do is to apply - if you don't get the Med Card, you may get the GP visit card instead.
You can get the application forms [broken link removed] along with details of where to send the completed form.