The research showed that although 70pc of people believe they have the condition under control, only half have changed their diet and just one-third are taking enough exercise.
Almost one in four admitted they forgot to take their medication at some point in the previous month and one in 20 lapse at least once a week.
Prof Seamus Sreenan, consultant diabetologist in Connolly Hospital, Dublin, warned: "I would be concerned that some have a false sense of security.
"Effective treatment of type 2 diabetes for most people requires a significant change in diet and exercise as well as constant monitoring.
"We as healthcare professionals caring for people with diabetes appreciate the difficulties involved in making significant lifestyle changes and are concerned that our advice is not always followed through."
The problem is that uncontrolled diabetes can have no obvious symptoms - and the complications only come to light when patients are assessed at diabetes clinics, he added.
"I would encourage all people living with type 2 diabetes to agree specific targets for sugar, blood pressure and cholesterol levels with their healthcare professional so that they engage fully to best control their condition."
The research, carried out by Diabetes Ireland with the drug firm Janssen, comes amid predictions that rising levels of obesity will see the numbers with the disease escalate.
It showed that more than one in four who are newly diagnosed are confused and find it difficult to understand the kind of lifestyle changes needed to live with the condition.
They need to keep blood sugar levels within a range recommended by their doctor. They need to reduce the amount of fat, salt and sugar they eat while increasing fibre. They must eat regularly, and include starchy carbohydrates.
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