Under new fees legislation affecting UK residents, students from Northern Ireland were expected to pay fees of up to £9000 a year to attend Scottish universities from this year.
However, if these students take up their right to dual citizenship with the Irish Republic they can apply as an EU student and have their fees paid.
The Scottish Government has played down the development, saying it would only apply to a small number of people.
However, Dundee University has recorded a 20% increase in the number of students who live in Northern Ireland applying with an Irish passport.
So far, the university has made 800 offers to students living in Northern Ireland and 150 have been assessed as EU students.
The university made a similar number of offers last year with hardly any using the EU route.
Other universities contacted by The Herald have not seen any similar trends, although several said it was too early to tell.
Liz Smith, education spokeswoman for the Scottish Conservatives, called on Education Secretary Michael Russell to close the loophole quickly.
"Fees from students across the UK provide significant revenue and the SNP has not been clear on how this shortfall can be made up. Aside from that, it is discriminatory," she said.
"I have written to Mr Russell because he has a responsibility to put in front of Parliament exactly what the policy is and explain the criteria for fees.
"This issue has to be cleared up urgently and Mr Russell can no longer use the defence that there is no evidence of this happening."
Jenny Marra, Labour Party MSP for Dundee, also demanded clarity.
"The SNP's fees arrangement for the rest of the UK is fast becoming a farce as predicted," she said. "Mike Russell needs to give the universities clarity so they know what they should do in this situation."
A Scottish Government spokesman said: "We closely monitor application trends and are prepared to act if it appears issues such as this are arising."
The development may damage a new private income stream for universities, with the costs picked up by the taxpayer. It could also mean greater competition for places in Scotland.
The impact is still unclear, however, with a legal question mark over whether a student's nationality takes precedence over where they live.
There may also be disadvantages for students applying under the EU route because numbers are tightly controlled.