Es una propuesta para dejar clara la contraposición entre las asignaturas que se pueden cursar a distintos niveles, que corresponden a calificaciones finales distintas (foundation, ordinary y higher), y las asignaturas para las cuales existe un único nivel (common). También se podría decir común, pero es una palabra más polisémica y por lo tanto, me parece, menos clara.

The prevalence of a common level for most subjects gives schools the opportunity to differentiate their teaching within classrooms to meet the needs of all students and to ensure that the focus is on motivating students for learning rather than for assessment as is the case currently. (http://www.education.ie/en/Publications/Policy-Reports/A-Fra...

Levels
At the Junior Certificate, students can take an examination subject at one of four levels. These are:
- Higher Level (Irish: Ardleibhéal; sometimes called "Honours") – available in all subjects except CSPE.
- Ordinary Level (Irish: Gnáthleibhéal; sometimes called "Pass") – an easier course than Higher Level; available in all subjects except CSPE.
- Foundation Level (Irish: Bonnleibhéal) – an easier course than Ordinary Level; available only in English, Irish and Mathematics.
- Common Level (Irish: Leibhéal Comónta) – available only in CSPE.
(from https://en.wikipedia.org/wiki/Junior_Certificate)

See also:
https://www.examinations.ie/?l=en&mc=ca&sc=sc