Over 58,000 people have been waiting over a year for a vital scan to diagnose a medical problem.
They include MRIs, ultrasounds and CT scans.
These new figures have been released by the National Treatment Purchase Fund (NTPF).
They show that, in the second quarter of the year, there were a total of 238,605 people on waiting lists for a scan.
This represents all outpatients who are waiting for urgent, semi-urgent, routine or planned scans.
Almost 39,000 have been waiting over 18 months - with 15,420 waiting over a year and a half for an MRI.
Galway GP Martin Daly is a former president of the Irish Medical Organisation (IMO).
"This has a major impact on the ability to achieve early diagnosis and intervention for patients; not just in GP surgeries, but also in hospital clinics," he said.
"The waiting list reflects the demand, and the lack of previous capacity in the system, for diagnostic imaging.
"It does need to be addressed.
"We are a first-world country, and people have first-world expectations of their health service," he added.
In total, almost one-quarter of those on the waiting list for a scan have been waiting over a year.
