If she is out of work sick, is she on disability? If she isn't, and she is looking for alternative work, she can get unemployment assistance/benefit. After that I'm not sure...
Have a look here for the entitlements re maternity benefit.
In order to qualify for maternity benefit, the claimant must be in work so it sounds like your sister won't qualify at the moment.
As terrysgirl said, your sister may qualify for illness benefit depending on how long she has been working altogether (how many PRSI contributions she has) - she can get a medical certificate from her GP and make a claim.
If she's not entitled to illness benefit, has no other source of income, and is not available for work (i.e. she can't sign on for jobseeker's benefit/allowance) she may be entitled to Supplementary Welfare Allowance (SWA) which she can claim from the Community Welfare Officer at her local health centre. SWA is a means-tested payment, so any other income (e.g. if your sister is living with a working husband/partner) will be assessed.
My own wife is in a similar situation. Is currently in receipt of Disability Allowance and just found out she is pregnant also. She obviously satisfied all the conditions for receiving DA (e.g. PRSI contributions etc.). My question is will she continue just to receive DA or will she receive some other form of benfit as a result of being pregnant. Thanks, apple1