In general, the free education scheme allows students spend up to six years at second level, three years at junior cycle and three at senior cycle. The senior cycle may comprise a transition year after the conclusion of junior cycle, followed by a two year leaving certificate programme. Schools may allow students repeat a year in certain defined circumstances.
There is no upper age limit for students at second level. Indeed, I actively encourage students who may, perhaps, have left school early to subsequently return with a view to completing one of the available programmes.