If only the Dublin area had large expanses of vacant land, it might be possible to build more housing without the lengthy commutes and all-day congestion that are inevitable with urban sprawl. Plans like the scheme to expand Athlone, 123km from the city, to a population of 100,000 would be dismissed, bound to make congestion worse and to encourage even higher car ownership.

But the impression has been created that there is no suitable land available closer to the city, so the towns of Leinster are the only candidates to absorb population growth. The transport consequences of urban sprawl will just have to be endured or expensive railway projects undertaken.

It turns out that the city has at least one huge vacant site, stretching to over 400ha or 1,700 acres, which has never been zoned for housing and which could accommodate 20,000 housing units and 50,000 extra people.

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The greenfield site is in the Dunsink area to the northwest of central Dublin, much of the land is already in public ownership and the location convenient to existing and planned public transport links.

These include Ashtown station on the Maynooth railway line, for which capacity expansion is in hand, and a new Luas tram already announced to nearby Finglas.

The site is entirely inside the M50 ring road which, for most of its length, skirts the city at a distance of no more than about 8km. Empty land inside the M50, it turns out, is more plentiful than you might think, but there is a shortage of one vital ingredient, planning permission.

If Dublin were largely contained inside a distance of 12km or 13km from the centre, inside or just outside the M50, urban sprawl would be discouraged, congestion reduced and housing rendered more affordable.

Opposition to the draft plan for Dunsink is not coming from people concerned about coherent urban and regional development.

Fingal County Council is the responsible local authority and is considering what to do. Since local authorities are the gate-keepers of land-use changes under the 1963 Planning and Development Act and its successors, the council has initiated a public consultation, local objectors have been lining up political opposition and WhatsApp groups are being mobilised by residents’ associations in the adjoining neighbourhoods.

Lawyers have been retained and the new suburb, even if the scheme eventually gets to be built, will take forever, possibly decades.

Fingal County Council is the one controlling the northern part of Co Dublin and its officials have sought to prevent the utilisation of the new runway at Dublin Airport, first planned 40 years ago.

The runway has already been built and central Government has stepped in to veto the council’s obstructionism but the housing plan is merely in draft.

The objections include the familiar “excessive density” and “visual intrusion” even though there would be limits on the height of apartment blocks and provision for public parks.

Other staples of the objection industry are also enjoying a run-out, including wildlife conservation.

Dublin is a notably low-density city and unless vacant land is identified and zoned for housing, especially in the most convenient locations, the urban sprawl will continue, as will the pressure for expensive urban rail investments, of which the MetroLink is the most recent example.

The latest guess at the cost of this single, and largely underground, railway line has been reported as €19bn, with almost the first billion already spent on plans and property acquisition. The cost could be six times the figure for the National Children’s Hospital, the biggest overshoot to date in State capital spending.

But MetroLink is just a single route northbound from the city centre to the airport and Swords, not a transport solution for Dublin.

The principal destination it serves, Dublin Airport, is just 9km from the city centre and well served by frequent scheduled buses from all around the city and from every county, north and south. The route of the MetroLink is roughly parallel to the Port Tunnel, already built, paid for, and popular with bus companies and their customers.

There is every chance that the end-to-end journey time to the airport on MetroLink will be no improvement on what is currently available with the buses.

Thus the projections for patronage on MetroLink include optimism about diversion from the buses. The €19bn figure is not official – the Minister for Transport has yet to give an up-to-date estimate and another monster overshoot is a legitimate concern.

Ironically, the new runway at Dublin is one of the few recent capital projects to have come in on budget.

If the Dunsink project goes ahead and the MetroLink is cancelled, it is not too late to stop the urban sprawl and protect State finances.