A new report shows HIV, syphilis and gonorrhoea are all on the rise in Ireland.
The preliminary data for the first six months of this year has been published by the HSE Health Protection Surveillance Centre (HPSC).
It shows rises in notifications of these STIs, particularly among men who have sex with men (MSM).
While the number of cases of most STIs remained relatively stable in 2015 compared to 2014.
Dr Derval Igoe, a specialist in public health medicine at the HPSC, said: "We are concerned about a 50% increase in syphilis, and a 30% increase in HIV cases in 2015.
"Although some of the increase in HIV can be explained by a change in the notification procedures for HIV and an outbreak in people who inject drugs, these increases have largely been seen in men who have sex with men".
This group account for four-fifths of the syphilis cases, and more than half of the HIV cases.
For HIV, an increasing proportion of men who have sex with men born abroad - particularly from Latin America - who were HIV positive before coming to Ireland has contributed to this.
There are also increases in the number of men from abroad who report acquiring HIV in Ireland.
Gonorrhoea up by 63%
Although gonorrhoea numbers in 2015 were similar to 2014, the latest data for 2016 show that gonorrhoea rates in men have risen by 63%.
"This has been seen only in men, suggesting that this increase is occurring mainly in MSM", Dr Igoe said.
A recent survey on sexual behaviours in MSM in Ireland reported that 25% of men have had condomless anal intercourse with more than one partner in the last year, posing a risk of acquiring or transmitting HIV and STIs.
While 37% of men had never been tested for HIV, and 38% had never been tested for an STI.
Dr Fiona Lyons, clinical lead for HSE’s Sexual Health and Crisis Pregnancy Programme, said: "Promoting condom use and regular testing remain key to limiting the spread of HIV and STIs in men who have sex with men".
Noel Sutton, director of the Gay Health Network, added: "Safer sex using condoms is an effective way of preventing HIV and other STIs.
"We encourage men who have sex with men and who have taken a sexual health risk to get tested for HIV and STIs."
Details of free HIV and STI testing services, condoms, support and information are available here