I'm using a broader definition of "left" than you. I think context is important here. Regardless of what anyone thinks of Labour, SF and some of the independents' general left-ness, objectively they are lefter than FF or FG and I think the fact that so much of the "anyone but Fianna Fáil" vote went to them rather than to FG or the right-wing independents should be considered a step forward for the left - even if only a marginal one. Also, it's very clear that a lot of the support for those candidates who you don't consider left came from people who *do* consider them left, and voted for them (or transferred to them) for that reason.
BTW, these pages are interesting: http://www.politicalcompass.org/ireland2011 http://www.politicalcompass.org/ireland
The fact that Labour's best ever result has gone hand-in-hand with a rightward move is, I acknowledge, better evidence for your argument than mine. But I don't believe most of those who voted for them for the first time did so for that reason.