The following is taken from the FIS information page on welfare.ie
Under the Maternity Protection Act 1994, a woman who qualifies for Maternity Benefit, Adoptive Benefit or Health and Safety Benefit is entitled to claim FIS. Your income must be less than the income limit for your family size. (If you are claiming Maternity Benefit your average weekly earnings are worked out by adding together the 26 weeks of Maternity Benefit and your earnings from work for the previous 26 weeks and then dividing the total figure by 52). Your FIS claim will then be paid for one year from the start of your Maternity Benefit payment). You are not entitled to continue to claim FIS if you take additional unpaid maternity or adoptive leave or if you do not return to work following maternity or adoptive leave.
You will be eligible for FIS while on Maternity Benefit, but if you decide not to return to work when your Maternity Benefit ends, you will no longer be eligible.