The difference is that the Social Democrats are a left-wing party.
Labour has been tainted by going very, very sleek and middle-class during the Celtic Tiger; then on the collapse of the economy, a Labour minister had the post of Social Welfare and made savage cuts affecting the poor, while people like bankers and developers who had bankrupted the country were sympathetically supported. The same Labour admin put notices on every Social Welfare page encouraging people to inform on their neighbours if they suspected them of "defrauding" the State by claiming social welfare payments they weren't entitled to - of course there was a flood of people grassing up single mothers, etc; most of these investigations came up with nothing - but Labour became known as the rat-'em-up party. This distrust has continued among those who would have been the main voting bloc for what originated as James Connolly's party. The distrust has begun to ease, or rather, a couple of the current Labour TDs are well liked, but I don't think they'd be thought of as a party that represents the less well off, even now.