242 cases were reported in the first four months of this year
There are concerns that there may be 'a large reservoir' of undiagnosed cases as many STI services have been operating at a reduced level due to the Covid-19 pandemic.
Untreated syphilis can cause serious health problems to the heart, brain, eyes and nervous system.
Dublin, Kildare and Wicklow accounted for 79% of cases, followed by 9% in Cork and Kerry.
91% were in men and 9% were in women.