The women's section of Limerick Prison has been an average of 39% above its capacity over the past month.
And the Dóchas Centre for female offenders in Dublin was an average of 12% above its limit.
The Irish Prison Service says new accommodation is being built in Limerick that will increase capacity for female prisoners.
"The vast majority of women in prison are serving short sentences for less serious offences," outlined Fiona Ni Chinnéide, from the Irish Penal Reform Trust.
"We are very concerned about the persistent overcrowding in both of the women's prisons in Ireland and the persistent number of women being sent to prison particularly for short sentences and detained on remand.
"What we saw in 2015, 2016 and 2017 that the numbers in the overall prison population dropped but this never happened with the female prison population."