The HSE is normally very good at providing a policy for the management of sick leave and it’s likely that it was provided to you on commencement. It should certainly be available from your HR Unit.
Without sight of the policy, it’s likely that if you weren’t back at work (for whatever reason) between the two sick absences, it is treated as a single absence.
The duration of the sick absence will be the period from the first sick day to the day before you resume. So, for example, if your first sick day was last Friday, and you go back to work tomorrow, this will be regarded as a 7 day absence (Friday to Thursday inclusive). As such, you’ll need a medical certificate.