Public patients, whose GP refers them for a diagnostic scan such as an X-ray or MRI, face a postcode lottery when it comes to speed of access.
They also endure much longer waits than a privately insured patient for most of these scans, a survey of GPs has found.
The scans can help explain why a patient is experiencing a particular symptom and give vital information on what could be a potentially serious illness.
The family doctors were asked to outline the waiting times for when they referred a patient through direct access, whereby they can request a scan without first going through a consultant.
The survey of almost 300 doctors, by the Irish College of General Practitioners, found that while a private patient can get an ultrasound scan in four days, a public patient would have to wait an average of 14 weeks.
The GPs said a public patient needing a CT scan would face a delay of 16 weeks compared to just 5.5 days if they had private cover. When it comes to an MRI scan, a public patient can wait 22 weeks while a private patient is seen within seven working days.
A dexa scan, to detect bone-density problems, can take 24 weeks for a public patient to access, while a private patient gets it in a week.
Despite promises by the last government, around 20pc of GPs do not have direct access to several forms of scan and it varies in different areas.
Around 20pc cannot refer their public patients directly to a hospital for either an abdominal or pelvic ultrasound.
Just 57pc have direct access to a colonoscopy, a test for bowel disease, including cancer. However, 85pc can make a direct referral if the patient is going private.
The divide between public and private is least when it comes to referral for an X-ray or injury.
The GPs admitted that the problems getting the scans on time mean they are often having to send the patient to a hospital emergency department in the hope they will get the test there.
The report said in all services, the wait for access to diagnostics for public patients is unacceptably long when compared to private patients.
"There is no doubt that as a result GPs are forced to refer patients to overcrowded emergency departments in order to access diagnostic tests.