Just as Clubman said - the person should apply for Illness Benefit, and submit medical certs if they're unavailable for work.
Entitlement to Illness Benefit is based on PRSI contributions, so if the person has been on Jobseeker's Allowance, they may not qualify for it.
If they don't qualify for Illness Benefit, the person may qualify for Supplementary Welfare Allowance, from the Community Welfare Officer at their local health centre. This is a means-tested payment rather then PRSI-based.