Ring the Revenue back and tell them that you were receiving casual jobseekers (I presume it was benefit, not assistance). Casual jobseekers' benefit is not taxable, though full time jobseekers' is. Unfortunately, the welfare IT system, which advises the Revenue of payments, does not differential between the two, you have to let them know which you got.
You will have to pay the income levy unless you had a full medical card during the year. The health levy will also apply if your weekly earnings exceed €500pw, but if your total earnings for the year are less than €26k, you can apply for a refund of the health levy.