A new survey has found that 40% of 17 to 18-years-olds have had oral sex, while 33% say they had sexual intercourse.
The data is part of the latest tranche of the 'Growing Up in Ireland' survey from the the Economic and Social Research Institute (ESRI).
Males were more likely to report being sexually active than females (45% compared to 39%).
Some 56% who reported having had sexual intercourse also reported that they always used a condom, although a sizeable minority of 11% said they never used one.
Further, 79% who had had sexual intercourse said that they or their partner always used some form of contraception, although 6% said that they never/hardly ever used any form of contraception.
Some 13% said that they felt a little pressure, and 4% said they felt a lot of pressure to have sex.
And 6% of those surveyed also said that they were afraid of losing a partner through not having sex with them.
It also found that the majority of teenagers, 89%, had consumed alcohol.
A minority (6%) of said they drank two to three times per week, 40% drank two to four times per month and 48% monthly or less.
In general, males tended to drink more often than females.
Smoking and drugs
While 51% said that they had never smoked, 12% said they smoked ‘occasionally’ and 8% said they smoked daily.
The ESRI said smoking was strongly related to family social group: 5% of young people from professional/managerial backgrounds said they smoked, compared with 17% of those from the most disadvantaged social group.
As with drinking alcohol, smoking from an earlier age was associated with heavier smoking by the age of 17/18 years - 31% of those who had smoked a cigarette by 13 years smoked daily by the time they were 17/18-years-old, compared with only 6% of those who had not smoked by 13 years.
Just over one-third had tried e-cigarettes - 40% of males compared with 28% of females.
When asked if they had ever used cannabis, 69% said they had not; 17% used it 'once or twice'; 4% said they 'used to use it but not now', 8% said they used it 'occasionally' and 2% said they used it 'more than once a week'.
James Williams, research professor at the ESRI, said: "The importance of the Growing Up in Ireland study is that it provides very detailed and often sensitive information from nationally representative samples of children in order to highlight the areas of young people’s lives where they most need support.
"A significant finding from the research published today is the importance of early interventions in behaviours that compromise health, such as smoking and alcohol consumption.
"Drinking alcohol or smoking at an early age is associated with more frequent and higher levels of consumption by the age of 17/18, which points to clear ways we can help teenagers to make healthier choices.
"The findings published today represent only a small part of the data available from the latest round of the study, which covers multiple aspects of young people’s lives and provides a valuable picture of people transitioning to adulthood in Ireland today."