That's inaccurate. The public sector doesn't build homes in this country and hasn't built for a long time. The private sector builds for the public sector one way or another. At best some parts of the public sector have professional staff who can form part of the design team, but it's far more usual that they write a specification which is used to tender for the design team.
It's been shown time and time again that the private sector extracts far higher prices from public sector contracts than from the equivalent private sector contracts. That's true of both housing and office- there's no way for example any contractor would have dared charge €440k for a glorified mobile home like they did for the OPW, because they'd never get another job again.
The real problem is that virtually none of the people "responsible" (quotation marks because it's blindingly obvious nobody is actually held responsible) for public works could manage the construction of an ikea wardrobe at a reasonable cost, never mind a residential or office development.
On the question of luxury apartments in D4 for social housing, I'm totally fine with that. (FWIW neither i nor any family member I'm aware of has ever benefited from social housing). Social housing benefits more it costs, and there's behavioral trends which suggests putting poor people in wealthy areas is probably the best use of social housing because the poor kids will have peers who expect to go to college, pay tax etc and people tend to conform. Ultimately the money comes back several times over to the taxpayer, it just takes a while. A residential property is a 100 year asset.
I also don't agree that there's any moral hazard to providing secure social housing. I think the maximum contribution is 16% of income, so you'll have to earn €75k a year to pay €12k annual rent in Dublin which would be an absolute bargain. It reduces the incentives attached to higher wages but that's true of every tax. You need to be truly bad at maths to think there's no financial benefit to earning €60k instead of €50k etc.
Selling social housing to tenants on the other hand is just stupid.