Breastfeeding rates in the Republic are improving faster than they are in the North.
58% of new mothers in the Republic breastfeed their babies, compared to 45% north of the border.
The widening gap is highlighted in an all-island report from the Institute of Public Health.
Policy director Dr Helen McAvoy said there were lots of resources out there to help mums.
"Go to breastfeeding.ie, where there's a range of useful information, including videos where you can listen to the perspective of other mothers (and) signposting to local support groups," she said.
Breastfeeding rates at discharge from hospital increased from 49% to 58% between 2006 and 2015 in the Republic.
However, just under 54% of babies were still receiving breast milk a few days later when the public health nurse visited their homes. At three months, just over a third (35%) of babies were still receiving breast milk.
Between 2006 and 2015 in Northern Ireland, breastfeeding rates at discharge increased by 5% — from 40% to 45%. However, just over one in five (21%) babies were receiving some breast milk at three months.
Younger mothers and those from a lower socio-economic group in both jurisdictions were less likely to breastfeed.
Older mothers and those from higher socio-economic groups were the most likely to start breastfeeding and continue with it.
Dr McAvoy said breastfeeding rates made a significant contribution to population health. Breastfeeding protected babies and contributed to maternal health.
The Health Service Executive wants 38% of all babies to be breastfed at three months of age between now and 2021.
Additional reporting by Evelyn Ring.