That's a great question, and one Stephen answered recently on Ireland AM, which you can watch here
Essentially, party leaders don't sit on committees as it's very difficult to have the time to do both well, and he became a party leader last year. His attendance in the first 4 years was great - and he made great contributions and tabled a lot of legislative amendments, but of the hundreds of amendments tabled by opposition TDs a whopping zero were accepted by the government.
It's a shame that our committees are much less useful than in other major democracies, and actually something we propose changing in our Manifesto, taking a lot of power away from the cabinet and transferring it to the committees and putting them at the heart of our legislative process:
With all respect, blaming the government for every negative thing that happens is the kind of petty politics I would have hoped the Social Democrats were above.
As the poster above said, if Stephen felt that he was unable to do the job that the people of Wicklow (myself included) put him there to do, he should have resigned and given the opportunity to someone else. He was elected to represent the views of the people - even if they are not being listened to - not to be leader of a party. Other opposition people seem to have managed to continue attending despite their amendments not being accepted.