Time to stop imposing PHD planning ideals on the Irish housing market....and time to start letting the free market serve the various housing needs out there.....where those housing needs are being expressed via bottoms up preferences i.e. market democracy versus central planners . . .
You can't have housing without central planning, unless you think that "market democracy" is going to lay on roads, transport links, water supply, power supply, sewage, etc. Also there's a bit of tension between your arguments in favour of "market democracy" and your proposal that people living in the "wrong" property should simply be taxed at a rate that will encourage them to move into the "right" property. It's not clear to me why we should use tax to force people into the housing we think they should be in, but not use planning and development laws to facilitate the provision of that housing.
As to the suggestion that the falling household size is a function of peole becoming wealthier, two thoughts occur to me:
First, it's possible that as a result of becoming wealthier people may aspire to live in smaller households, but it doesn't follow that they have become sufficiently wealthier to actually pay for that (because it may cost a lot more than they expect it to). That would help to explain why people can't afford the houses they think they ought to have.
Secondly, and probably the bigger point: the decline in average household size is not mainly about becoming wealthier; it's about later marriage and smaller family sizes. So, relative to a generation ago, we need a lot more accommodation suitable for single people (because we are single for longer) but we don't require as many big family houses (because we don't have big families).
Another factor that's at work here is the impact of rapid population growth. Ireland used to have a relatively low population density; we are now almost exactly at the European average for population density. We also used to have a relatively dispersed population; we are now pretty much at the EU average for urbanisation.
All-in-all, I suspect our aspirations and expectations with regard to housing are conditioned by history, by culture and by memory of past conditions and experiences more than they are by current realities. We want a housing stock that is like the housing stock of a generation or two ago but of a higher standard and more of it. But for a variety of reasons that's not the housing stock we most need and actually providing the housing stock we want would be inefficient, in a context in which we pay a very high price for inefficiency.
Ther may be a choice to be made between:
(a) providing housing units of the type that people aspire to, but nothing like enough of them and at a very high price; or
(b) providing a greater number of housing units at a more affordable cost that are a better fit to the relevant demographic and other factors, but that people will feel somewhat dissatisfied with.