Up to 17% of people who have had sex with someone outside of a steady relationship did not use any form of protection.
The results of a survey carried out by Healthy Ireland are being launched today as part of a new campaign urging people to ask their pharmacist for advice about their sexual health.
The number of sexually transmitted infections has risen significantly in the past number of years.
People are being reminded that their pharmacists can offer private consultations and can arrange for STI testing.
IPU President Kathy Maher said: "The incidence of STIs is on the rise and it is critical that people take steps to protect their health and get tested.
"Your pharmacist can also offer advice on contraception, as well as dispensing emergency contraception if required."
Ms Maher also said that "80% of STIs are symptomless".
"For that reason, even if you feel healthy, if you have had unprotected sex, you should ask your pharmacist for advice about getting tested. Early diagnosis is critical in helping to prevent the spread of STIs, as well as maximising the health outcomes of the infected person."
A list of free STI testing clinics can be found on www.hivireland.ie.