Data provided by Women on the Web, which offers abortion pills online, showed 5,650 women from both countries accessed the service from 2010 to 2015.
Most already have at least one child and a majority said taking the pills had been the right choice for them.
Laws in both countries ban or restrict access to abortion.
Abortion is legal in both countries if the mother's life it at risk. In Northern Ireland, it may also be permitted to preserve her physical or mental health.
Accessing pills which will terminate a pregnancy, or helping others to do so, can lead to a criminal prosecution in Northern Ireland, with the maximum sentence being life imprisonment.
Protesters are calling for a referendum to repeal the laws in the Republic. Later this month, Prime Minister Enda Kenny will hold a citizens' assembly to discuss whether a vote should take place to change abortion laws.
Since 2001, the number of women travelling to England and Wales for abortion services has fallen.
'Could not cope'
In a paper published in the British Journal of Obstetrics and Gynaecology (RCOG), researchers from the University of Texas in Austin in the US analysed information provided by women using Women on the Web.
They say it is the first picture of women who obtain abortion pills online.
Of the 5,650 women who used the service, 25.8% were aged between 30 and 34 and 24.1% were aged from 25 to 29.
The lowest percentages were among women under 20, who made up 4.6% of those using the service, and over 45s who accounted for 2.6%.
Three-quarters were seven weeks pregnant or less.
Data from 3,500 women who used the site between 2013 and 2015 found slightly under two-thirds already had one or more children.
The most common reason for terminating their pregnancy was that they could not cope with a child "at this point in my life".
Having "no money to raise a child" was the second most common reason, followed by a feeling that their families were complete.
Contraceptive failure was the most common reason cited for pregnancy.
Prof Abigail Aiken, who led the study, said: "The findings of this paper contribute new and important evidence to the abortion policy debate in Northern Ireland.
"Northern Irish women have described in their own words the benefits of access to safe early medical abortion for their health, wellbeing, and autonomy. The current abortion law, which dates back to 1861, harms women by creating a climate of stigma, shame, and isolation."
However, Bernadette Smyth, of the pro-life group Precious Life, said: "Abortion is a criminal offence in Ireland and if women are accessing these dangerous pills online and have an adverse reaction that could endanger their life, who will take responsibility?
"To ensure the safety of women and babies these illegal online sites must be closed down.
"I want women with crisis pregnancies to be fully informed of the dangers of these pills and, more importantly, I want to provide help and support, because abortion is not the answer."