Why don't the Labour Party and the Social Democrats merge?

Because the two until recently SD co-leaders, who retain huge influence, would never countenance it; basically.

When they eventually retire (they have their Dáil seats for life, realistically) it may be possible.

There'd also be significant issues with deciding on a name - the Soc Dem name hasn't got baggage but Labour would likely still be the bigger party in terms of both public reps and members 2:1 or so; and a bigger geographic spread of where those reps and members are.

Ah Raoul you know as well as anybody that a meeting of these left leaning parties always ends up like a scene from a Monty Python movie.

Would pay to see their first meeting live streamed though.

There's also a reason all the current SDs never joined Labour and instead joined the SDs. They didn't do it because they didn't realise Lavour existed. For various reasons they didn't feel Labour aligned with their views. The same for SD voters. They vote for the SD because they feel Labour dont/didn't aligned with their views. That's still something that would need to be considered and whether SD members would vote for a mearge.

But you have basically said it is being stopped by two big personalities in the SDs. Why did Hilly Cairns join SD and not Labour?

There's no need. But I have a feeling the reason isn't that she was forced to by the two big personalities.

Labours purse strings are still controlled by the ex worker's party types, and that's a turn off. They would fall out with a stick.

Personality types as well, Labour are more old school refined, where as the Social Democrats are a more on trend type.

In a normal circumstance they would merge as the differences are small but they won't.

The SD are the people who would be labour youth

As I said, there's a reason she, and the others, joined the SD. I just find it strange that those reasons wouldnt also exclude a mearger with Labour.

Shortall has to go.

Not a hope! The two former co-leaders are on the pig's back. Both have the full State Contributory Pension plus TD's salaries and expenses. (Shortie might have a Junior Minister's pension too, if she was a junior for long enough - I don't know if that's paid when a TD reaches 66)

If they retire, they'll be left with nothing to do except reading the papers, cutting the grass and minding the grandchildren.

Seems odd to want to merge them. What are the long term pro's, this new left party ends up being the junior party in government and drifts to the centre. Then gets obliterated in the following election and there is no alternative other than the greens to centre left voters. Rinse repeat.

It's good and healthy to have choice, they can grow in a healthy separate manner, and differentiate on policy in small ways to give people healthy choice.

I think the calculations of merge Lab and SD and 1+1=2, wouldn't work out as well as people think. More than likely it will be 1+1=1.5 at best, in terms of support

Or LSD Party?

They have great vision(s)