I'm guessing you're on Jobseeker's Allowance (means-tested), rather than Jobseeker's Benefit (based on PRSI, paid for a max of 12 months).
The reduction will depend on her earnings.
1. Calculate her daily earnings - gross weekly pay less PRSI, union, superannuation, PRSA, AVC divided by the number of days worked
2. Calculate the daily means by subtracting €20 from the daily earnings figure, and multiplying the result by 60%
3. Multiply the daily means by the number of days worked (3) - this is the weekly means
4. Subtract the weekly means from your JA payment.