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The jobless rate compares to 10.2 percent in December 2017.
The INE revised its original estimate for November 2017 from 8.2 percent and said the rate was the lowest since November 2004.
The youth unemployment rate of those aged 15 to 24 fell to 22.1 percent in December from 22.8 percent in November and from 26.2 percent a year earlier.
The total number of unemployed fell to 401,500 people last month from 417,200 in November, while the employed workforce stood at nearly 4.78 million.