Sign on initially anyway for Jobseekers Benefit (not allowance as presume you have sufficient PRSI contributions for benefit), you may as previous poster says be disqualified for up to 9 weeks for voluntarily leaving a job, this is discretionary and it will depend on why you left etc.
I would certainly not be saying to SW at this stage what your plans may or may not be other than you are looking for work, as presumably you are and would take a job if you got one.
If once your claim is processed you then decide to become self employed you can apply for Short Term Enterprise Allowance, this basically converts the Jobseekers Benefit to STEA, same amount and runs for same amount of time but means you can commence self employment while still getting your benefit paid.
To apply for this you will have to meet the co-ordinator of the scheme in your local SW office and produce a business plan etc.
I would also recommend a trip to your local Citizens Information Office to discuss your options.