Gender figures: Migration into Ireland has contributed to an overall excess in the number of males compared to females in the State.
Figures show that there tend to be more females in cities and urban areas, while there are more males in rural counties such as Leitrim, Cavan and Roscommon.
CSO officials say that the high level of male migration into the Republic is the main factor behind the change.
For example, the tens of thousands of migrant workers from former EU accession states who have moved here in recent years are predominantly young and male.
Ulster has the highest male excess in the country (with 1,027 males per 1,000 females), followed by Connacht (1,019 males per 1,000 females) and Munster (1,010 males per 1,000 females).
Leinster is the only province where there are more females than males (989 males per 1,000 females).
The overall figure for the country is 1,001 males per 1,000 females.
These latest figures reverse a trend of the last 20 years which has seen females outnumber males.
The 1996 and 2002 census figures, for example, recorded the highest ever ration of females to males, with 986 males per 1,000 females.
In 2002, Ulster also had the highest male excess in the country (1,024 per 1,000 females), followed by Connacht (1,006 per 1,000 females).
However, women outnumbered men significantly in Leinster (972 males per 1,000 females) and to a lesser extent in Munster (998 males per 1,000 females).
The excess of men compared to women is a return to a trend which prevailed in Ireland between 1926 and the early 1980s.
Statisticians and sociologists say that this imbalance was linked to emigration and the agriculture-based economy, where a son was more likely to remain at home.
Males outnumbered females most starkly in the 1926 census, when there were 1,029 males for every 1,000 females.
The proportion of females began to increase steadily between 1926 and the 1980s. Females outnumbered males for the first time in the 1986 census.
This trend continued up to the 2002 census, with the imbalance between females and males growing with each census.