Eoin Ó Broin (Dublin Mid West, Sinn Fein) | Oireachtas source

It continues:

Notwithstanding subsection (10), where a modification is likely to have a significant effect on the environment, or is likely to have a significant effect on any European site either individually or in combination with other plans or projects, the modification shall not be deemed to be minor.

Sometimes, impacts are cumulative and relate to adjacent sites, projects or plans. Therefore, they need to be viewed cumulatively rather than individually. That is the logic of the amendment.