Do you mean qualifying for mat leave or benefit - qualification for benefit is dependent upon prsi payments, qualification for leave is dependent on working (getting a prsi class A stamp) at some point after 24 weeks pregnancy.
If it's the latter then could you explain this to your company and see if they could accomodate you with a later official finishing date, maybe by adding your hols, as you mentioned. Discrimination laws aside, it's not easy for a visibly pregnant woman to get a job.