To qualify for Jobseeker's Benefit (based on PRSI contributions) you must have 52 paid PRSI contributions since starting work
and
39 paid or credited PRSI contributions in the relevant tax year (for 2008, the relevant tax year is 2006, in 2009 it'll be 2007).
If you don't have 39 paid or credited in the relevant tax year, you may quailfy if you have 26 paid in the relevant tax year and 26 paid in the year before that.
Only PRSI contributions at class A, H and P count.
If you've been paying 51 A contributions for the past few years, then you should qualify.
Jobseeker's Allowance is means-tested, for persons who don't qualify for JB. Entitlement to that depends on your circumstances (savings, household income, etc).