The number of students from the Republic of Ireland studying in Northern Ireland has fallen by more than a third since 2011.
However, there has been a rise in the number of Northern Irish students in the Republic in recent years.
The findings are part of a joint government analysis of cross-border student enrolments.
It warns that Brexit could have a substantial effect on the cross-border flow of higher education students.
The analysis of student flows between the Republic of Ireland and Northern Ireland in higher education has just been published jointly by the Republic's Department of Education and Skills (DES) and Northern Ireland's Department for the Economy (DfE).
It shows that the number of students from the Republic of Ireland studying at higher education institutions in Northern Ireland fell from 3,520 in 2011/12 to 2,195 in 2015/16 - a 38% decrease.
Separate figures from the DfE show there was a further fall to 2,095 students in 2016/17.
As a result, students from the Republic of Ireland now make up only 3.8% of higher education enrolments in Northern Ireland.
The most recent figures from the Higher Education Authority (HEA) show that there were 1,328 Northern Ireland students at universities, colleges and institutes of technology in the Republic in 2017/18.
That is a rise from 970 in 2011/12 - a 37% increase.
However, with around 235,000 higher education enrolments in the Republic in 2017/18, Northern Irish students make up just over half a per cent of the student body.
Individually, countries like Canada, USA, India, China and Malaysia have many more students in the Republic than Northern Ireland.
Nine hundred and one of the Northern Irish students in 2017/18 were at universities - 794 full-time and 107 part-time.
The majority of the rest were enrolled at institutes of technology.
However, the analysis warns that Northern Irish students crossing the border may be liable for higher tuition fees after the UK exits the European Union.
"NI students coming to ROI Higher Education Institutions may in the future be liable for non-EU fees, which can be considerable," the report said.
Currently, for instance, Northern Irish undergraduate students pay a student contribution of 3,000 euros a year if they study in the Republic.
That is exactly the same as students from the Republic of Ireland, and they are eligible for a student loan to cover this fee and also a maintenance loan.
However, if they were treated as non-EU students after Brexit their undergraduate tuition fees could rise to around 19,000 euros a year or more.
A determination by the DES is yet to be made on how Northern Irish students will be classified in terms of fees in future.