If you were already in Ireland and managed to score 0% in every single component of the Junior Certificate you would still progress to the 4th year AUTOMATICALLY. (i.e. you can't 'fail' the examination and be forced to retake the third year; you MUST go on to the 4th year).
So even if a school (very unwisely) regarded a pupil who'd not actually taken the Junior Certificate examinations as having achieved 0% in all of them, that pupil must still be placed into the 4th year.
In practice, all schools (throughout Europe and beyond) are used to accepting pupils from other educational systems and use whatever input they can get from pupils' previous schools to place them appropriately within the system.
For example, many Irish schools might not enter pupils at Higher Level for the Leaving Certificate examinations unless they'd achieved at least a 'C' grade at Higher Level for the Junior Certificate. However if a pupil transfers into their school from another educational system (such as that in Scotland), with their former school reporting that they were in the top set for all subjects and expected to go on to achieve great things at university, the information from the former school would simply be used instead of grades at Junior Certificate level.