A newborn baby and four children were among the 395 people newly diagnosed with HIV last year, it was disclosed today.
Latest figures also showed a 42.2pc rise in the number of gay and bisexual men infected in 2009, despite the overall rate dropping slightly.
The Health Protection Surveillance Centre (HPSC) confirmed 138 men who had sex with men (MSM) were diagnosed up to December, compared with 97 the previous year.
Dr Aidan O'Hora, specialist in public health medicine, said: "The majority of these men -- 63pc -- were born in Ireland and most likely acquired their infection here.
"Young men under 30 years of age accounted for 35pc of new diagnoses. This trend is consistent with what is being seen in many other western industrialised countries."
The HPSC said 1,038 people were living with Aids by the end of December. The figure included 33 who were newly diagnosed throughout the year and two people had died from Aids.
A further 5,637 people were living with HIV.
But Dr O'Hora warned that due to the voluntary nature of the reporting system for HIV, it was likely that the number of cases was "an underestimate."
Of those newly diagnosed with HIV:
Tiernan Brady, of the Gay and Lesbian Equality Network (GLEN), said: "A renewed focus on prevention of HIV is urgently required with this group of men."
"The reasons for this increase must be better understood so as to reduce this rising incidence of HIV," he added.
Meanwhile, Dr O'Hora urged adults to practise safe sex.
"Anyone engaging in sexual activity should practise safe sex. A properly used condom provides effective protection from HIV," he said.