There has been a 22% increase in the number of people treated for syphilis.
Health chiefs revealed there had been a significant rise in the disease over the course of last year and warned that the final number is likely to be higher when the files of more patients are reviewed in the coming months.
The Health Protection Surveillance Centre (HPSC) said four out of five cases were reported among men who have sex with other men.
Its report revealed there were 249 cases of early infectious syphilis last year - only 10 of those involved women and more than half were in people aged under 34.
The data from the Health Service Executive's HPSC unit, which was published on its website, also showed infection rates in the east of the country ran at about 10 in every 100,000 people - around twice the national rate.
The most common sexually transmitted disease is still chlamydia, which can cause infertility, with 6,815 cases reported last year.
There were 1,305 cases of gonorrhoea, a slight decrease, and 1,275 cases of genital herpes, a 13% increase, the report said.
The HPSC also warned that young people and men who have sex with men continue to bear the burden of sexually transmitted infections.
It said that half of the recorded chlamydia cases were among those aged 15-24 and more than 70% of cases of gonorrhoea, lymphgranuloma venereum and early infectious syphilis, where mode of transmission was recorded, were among sexually active gay men.
"Safer sex practices and regular testing are key to the prevention," the HPSC said.
The HPSC also noted that the first outbreak in Ireland of sexually transmitted enteric infection involving contact with bacteria from the intestine or faeces was reported last November with 32 cases linked to it.