The Irish population could soar back to its pre-Famine level within decades.
New figures from the Republic's Central Statistics Office (CSO) indicate that the number of children and old people will grow particularly fast, putting pressure on the Irish government to deliver extra schools and health services.
A continued baby boom means there will be up to 100,000 additional primary school children by 2021, an increase of 20pc on today.
The number of elderly over 65 will also rise dramatically, from 532,000 now to over 850,000 by 2026 and close to 1.4 million by 2046.
The population of people aged 80 or more will treble from 128,000 to over 484,000 by 2046.
In total, the Republic's population, which was 4.58 million in the 2011 Census, will rise to between five million and 6.7 million by 2046.
If it reaches 6.7 million, that would exceed the 1841 highpoint of 6.5 million in the 26 counties just before the Famine.
The figures are contained in a CSO report Population and Labour Force Projections 2016-2046 published yesterday.
The reason for the wide variation in population estimates is that it all depends on what happens with inward migration.
Currently, more people are leaving the country than arriving -- there was net emigration of 34,500 people last year.
According to the CSO, migration flow can be very volatile and is influenced by the economy both at home and abroad, but the most optimistic scenario would see a return to net inward migration by 2016.
The gloomiest picture would see emigration exceeding immigration until 2021 and continuing at a low level after that.
The Republic of Ireland continues to have the highest fertility rates in the EU at 2.1 children per woman on average, though this could fall slightly.
The biggest factor influencing the number of babies born will be the number of women of childbearing age in the country -- again highly dependent on the levels of migration.
Life expectancy will continue to grow, increasing from 77.9 to 85.1 for men and from 82.7 to 88.5 for women by 2046.
The CSO projections are very important for long-term planning by state bodies to prepare vital services such as healthcare and education for increased demand.
Nursing Homes Ireland said the Irish government must set up forum on long-term care urgently to plan for the increase in the numbers of very old people.