THE Government will be forced to draw up new spending plans, after the census revealed a massive spike in the population.
Census 2011 published yesterday shows there are now 4,581,269 people living in the State, the highest figure since 1851, and almost 110,000 more than expected.
And the increase means that budget plans for next year will have to be re-drawn to account for major increases in the populations of some counties.
The Department of Public Expenditure and Reform last night said funding would have to be directed to regions which had experienced the biggest population growth.
Money will have to be found to fund schools, hospitals, roads, public transport and other essential services at a time when the economy is on its knees.
"The budget for this year has been set so it won't change those allocations," a spokeswoman said.
"The regional figures will help set out where that money will go.
"We would expect the consolidation as outlined in the EU/IMF programme to remain the same.
"We have assessments of annual population, and ongoing regular figures from the Central Statistics Office (CSO) in relation to the live register, so we do have indications and it (the census) will feed into the budget."
The department confirmed that €3.6bn would still be cut from next year's budget under the terms of the EU/IMF bailout.
But the population increase will have a major bearing on where schools are provided, on public-transport services in areas where the population has boomed and on the provision of frontline healthcare services.
The CSO said most of the population increase was down to a natural change in population -- the number of births minus the number of deaths -- fuelled by our high birth rate, which is the highest in the EU.
Some 363,500 babies were born between 2006 and 2011. In 2008, the last year for which figures are available, the rate of natural change was 10.4 per 1,000 population.
CSO director general Gerry O'Hanlon said the agency expected the population to continue to increase over the coming years, adding to the pressure on the public finances.
"The growth in population has shifted from (being fed by) migration to natural growth," he said.
"All population estimates are subject to revision, but we were somewhat surprised (at the increase).
"It was at the upper end of our estimates. We expected a 6pc increase, not 8.1pc.
"Between 2002 and 2006, migration represented 60pc of the change in population. Between 2006 and 2011, it's natural change 65pc, and 35pc migration.
"We would still expect a natural increase (over the coming years), but the big imponderable is migration.
"We're having net immigration at the moment and it's likely to be the same for the next few years."
The census data is based on forms collected last April and shows the population has grown by an average of 1.6pc each year.
Some 118,650 more people came to Ireland between 2006 and 2011 than left the country, but this trend was reversed in the latter half of the five-year period, with more people leaving than arriving.
The results show Co Laois had the fastest-growing population, increasing by 13,399 from 67,059 in 2006 to 80,458 in 2001 -- a rise of 20pc.
Falls were recorded in Cork and Limerick cities.
There are also more women than men in the State, with 981 males per 1,000 women.
The midlands is the only region to show more males than females, with 1,002 for every 1,000.
The census also shows that there are more than two million houses in the State, and that almost one in seven is empty.
The biggest rise in house numbers was in Co Laois (up 21.2pc), followed by counties Cavan, Donegal, Leitrim and Longford, which all had increases of more than 19pc.
The number of vacant units is higher than previously estimated, with the legacy of bad planning seen in rural counties like Leitrim, where one in three (30.4pc) of all houses are empty. It is followed by counties Donegal (28.5pc), Kerry (26.5pc) and Mayo (24.8pc).
In all, some 294,202 homes are empty, but the population increase may help to shift unsold units when the economy improves.