Future Forty, the long-horizon document released by the Department of Finance early last month, predicts that population could reach almost 7m by the year 2065, but with a huge range of variation in the forecast figure. There are scenarios which could see far lower, and far higher, out-turns for the population figure, likely to exceed 5.5m when the next census is taken in April.

The general reaction to the document has been political encouragement for accelerated State spending on infrastructure. It is salutary to recall the long-term population forecasts for Ireland made down the decades and the reliability of the predicted figures for long-term planning.

The best summary is that the forecasts were not very good, regularly pushed off course by the underlying factors which drive Ireland’s demographics. These are mortality, falling in line with better life expectancy, fertility, reflecting smaller families and migration.

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Both mortality and fertility rates change slowly from year to year and there have been clear trends which were easy enough to extrapolate. But migration is extraordinarily volatile and the forecasters, the present writer included, have often been wrong by large margins.

Net outward migration was enormous in the 1950s, up to 3% of the labour force exiting in some years, but inward flows began to out-number emigrants in subsequent decades. This pattern was interrupted on several occasions and the migration channel swamped the mortality and fertility factors, turning population growth into decline unpredictably, for example during the downturns after the 1970s oil shock and the budget cutbacks in the late 1980s, repeated 20 years later after the banking bust.

In between there was a strong inward flow of migrants, including returning natives, during the bubble period up to 2008, followed by several years of net outflows. The current economic recovery, however long it lasts, has seen a resumption of substantial net inflows.

It is understandable that politicians should focus on infrastructure shortfalls in these circumstances but there is insufficient attention paid to the structural factors. The Department of Finance authors echo earlier projections from the Central Statistics Office in highlighting the changing age profile of the population, in particular the steady ageing which has been under way for a generation, exacerbated by slower births and dwindling school-age populations in many areas.

Even if net migration turned out to be zero, there will be an adverse shift, if people continue to retire relatively early, in the ratio of tax-paying workers to those relying on pension income.

In Ireland a substantial portion of retirement income comes from the State budget, through old-age pensions via the Social Insurance Fund and unfunded occupational pensions payable to many categories of public employees. It is easy to show that the Social Insurance Fund will have a huge yearly deficit in due course as revenue stalls while outgoings move briskly ahead.

There is in reality no Social Insurance Fund – it is essentially a book entry in government accounts and there are no income-producing assets to help meet the deficit.

The same is true of the unfunded, non-contributory occupational pensions across large parts of public employment. The trouble with pay-as-you-go transfers is that they do not automatically stay in balance and the adverse trends which worry the Department of Finance officials will persist even if inward net migration were no longer positive.

If people continue to retire in their mid-60s and so stop paying tax, but decline to expire for another couple of decades on average, the sums no longer add up as they used to do.

The implication is that the State will have to find revenue elsewhere, and fairly soon if the corporation tax bonanza begins to dry up. The ageing problem is not unique to Ireland – the same mortality and fertility trends are evident elsewhere and are even more acute in France, where retirement ages are lower than in Ireland and the State balance sheet even more vulnerable.

Attempts to increase retirement ages have threatened the survival of the party political system and brought down prime ministers. In Ireland there is evidence that public attitudes are beginning to change.

There has been a significant increase in labour force participation in the older age groups, especially amongst women, reflecting both smaller families and higher completion rates in school and college. But public policy has been slow to follow.

The qualification age for the old-age pension is now 66 but there is a one-year scheme for those aged 65 provided they have stopped working. Some political parties urge the reintroduction of the automatic full pension at 65.

As people voluntarily choose to extend their working lives, the State could be offering some broad hints via the tax and social welfare system. Tax breaks, like exemption from payroll taxes for older people, are getting harder to justify.