If your GP decides that you are unfit for work, and will give you medical certs for SW you can apply for Illness Benefit - entitlement is based on PRSI contributions from work and credits from the dates you've been signing on.
Here's the info from SW website:
If you are getting long-term full rate Jobseeker's Allowance (15 months) and become ill, you can get the maximum rate of Illness Benefit. You must have at least 260 PRSI contributions paid at any time and at least 39 credited in the Relevant Tax Year. You do not need to have 13 PRSI contributions recently paid. Since January 2009, if you have 260 or more PRSI contributions you can get Illness Benefit for a maximum of 2 years.
If you are getting long-term Jobseeker's Allowance (15 months) and have at least 104 PRSI paid contributions since you first started work and at least 39 credited in the Relevant Tax Year, you don't need to have 13 PRSI contributions recently paid to qualify for Illness Benefit. You may not however qualify for the maximum rate of benefit. The maximum period of time you can claim for is 52 weeks.
The Relevant Tax Year is 2008 for claims made in 2010.