Some third-level colleges are delaying the entry of first years by a further week after confirmation that Leaving Cert results and CAO offers won't now be released until September.
The country's largest university, University College Dublin (UCD), said it would affect its plans to start Orientation Week for first years on September 14.
"We anticipate it will impact on these students and are seeking to minimise this for them," a UCD spokesperson said.
At Trinity College Dublin, Freshers' Week is being put back a week to September 28, with the start of the first-year teaching term also back a week, to October 5.
Technological University Dublin (TU Dublin) is delaying Orientation Week to September 28, with lectures for first years starting on October 5.
Dublin City University, which had planned a two-week orientation programme for first years starting September 21, said it would have to curtail that.
The disruption to the 2020 Leaving Cert prompted all third-level colleges to pencil in later starting dates, while the publication this week of the September timeline for the results and CAO offers forced a further review.
Maynooth University and the University of Limerick are sticking with their original reopening schedules.
Maynooth's Orientation Week will start on September 21, with lectures beginning the following week.
Limerick is going ahead with its existing plans to bring first years on to campus on September 28.
Leaving Cert results will issue on September 7, three weeks later than usual, and CAO round one offers will follow on September 11. Students will have until September 16 to accept an offer.
Round two offers will issue on September 23 and, depending on the college, acceptance of a place may mean a slightly later start for those students.
The late release of the results and offers has sparked some criticism, with Irish Universities Association Director General Jim Miley expressing "surprise and disappointment with the timing of finalising grades, noting in particular the challenges faced by students and their families".
However, Mr Miley did acknowledge "the unique challenges" facing the Department of Education, which, because of the Covid-19 public health threat, replaced the traditional June exams with a system of calculated grades.
He said the focus of the universities now was "to give clarity to students as early as possible. Each university will quickly finalise their revised plans based on the delayed date for grades release".
The Technological Higher Education Association, the umbrella body for institutes of technology and TU Dublin, said its colleges were committed to affording incoming students the necessary time to make arrangements.
Union of Students in Ireland President Lorna Fitzpatrick said the delay in issuing Leaving Cert results was unfair on students hoping to start third level, who had already faced so much change and uncertainty.
"The turnaround time between the results coming out, CAO and UCAS offers being made and college starting is now extremely short, which heaps yet more pressure on students," she said.
Further and Higher Education Minister Simon Harris moved to assure students that grant payments would not be affected by the delay and said Susi remained open to applications.
Meanwhile, the CAO offer season kicks off today with offers to 5,800 applicants, mainly mature students, mature nursing/midwifery applicants, and deferred applicants.