ONE in seven baby deaths at or around the time of birth in 2013 were either "unexplained or unspecified," according a new report.
The report said 436 baby deaths were reported during the year with severe birth defects accounting for one third.
One in five were due to maternal illness, but as many as 13.1pc were unexplained.
"The fact that just over 57pc of all perinatal deaths in Ireland do not undergo post-mortem examinations may be a contributing factor to the numbers assigned to this category," said the National Perinatal Statistic Report for 2013 published by the HSE.
Perinatal ranges from 22 weeks gestation to the first week of life.
The report said the death rate for babies at or around the time of birth had fallen from 8.1 per 1,000 live births and stillbirths in 2004 to 6.3 per 1,000 in 2013.
"This represents a 22.2pc decrease over the decade.
"In 2013, over 36pc of total births were to mothers aged 30 to 34 years, representing over one third of all mothers having either a single or multiple birth."
The perinatal death rate was highest, at 9.1 per 1,000 live births and stillbirths, for babies born to mothers age 40 to 44 years.
The lowest perinatal mortality rate of five per 1,000 live births and stillbirths was for babies born to mothers aged 30 to 34 years.
There were 69,267 births in 2013, a 3.8pc decrease from 2012.
It said that 56pc of babies were breastfed at some stage, compared to 53pc in 2009 and 46pc in 2004. Less than half were exclusively breastfed, only a slight improvement on other years.
Prof Michael Turner, National Clinical Lead, Obstetrics and Gynaecology, stated: "While there was a small decrease in the total number of births nationally from 2012 to 2013, the 10-year trends in this detailed report are significant.
"The total annual number of births has increased by 11pc.
"The average age of mothers has increased by 1.3 years to 32.1 years. The number of first-time mothers has increased to 38pc," he added.
"The number of twin births has increased by 24pc, and the number of mothers born outside Ireland has increased by 27pc.
"All of these trends bring increased demands on our maternity services nationally.
"In the light of these increased demands, it is reassuring that there have been improvements in the rate of stillbirths and neonatal deaths.
"It is also reassuring that while Caesarean section rates increased over the decade, they remain close to the OECD average for developed countries."
The rate of Caesarean section births continues to rise. In 2004, some 25.2pc babies were born by Caesarean section compared to 29.7pc for 2013.
The report showed:
The average age of mothers has increased from 30.8 years in 2004 to 32.1 years in 2013.
32pc of mothers were aged 35 years or older, up from 24pc in 2004.
20pc of first births were to women aged 35 years or older, compared to 13pc in 2004.
2pc of total mothers giving birth were aged under 20 years, compared to 4pc in 2004.
38pc gave birth for the first time, with an average age for first-time mothers of 30.3 years.
2pc had a multiple pregnancy.
56pc of mothers recorded any breastfeeding, compared to 53pc in 2009 and 42pc in 2004.
Women who had one baby stayed in hospital for two days or less and 44pc were there for three to five days.
A minority of women who had more than one child stayed in hospital for two days or less and 74pc were in hospital three to five days.
Nearly one in four births were to women born outside Ireland, compared to 15pc in 2004.
Recently published figures for 2014 by the HSE showed that the number of babies born across the country continued to fall in the majority of counties with only the main maternity hospitals in Dublin showing a modest increase.
Outside of Dublin, the only hospital which showed a small increase was Mayo General, which bucked the trend with a rise of 2.8pc in births.
End-of-year figures showed 67,347 babies were born in 2014.
This compares with a peak of 76,021 in 2009 as emigration and the recession were in full flow.
Among the hospitals which saw a drop in births was Portlaoise, where five babies died in controversial circumstances between 2006 and 2012.
Other hospitals to suffer a significant drop in births were South Tipperary (8.5pc), Sligo (9pc), Cavan General (7.5pc), Our Lady of Lourdes Drogheda (7.2pc) and Letterkenny Hospital (6.3pc).