HIQA says there has been a significant increase in the use of the practice since last year – and it's not always used as a last resort.
HIQA inspectors carried out an unannounced follow up inspection at Oberstown in June.
They looked at a number of things – but their biggest concern relates to the used of single separation.
Young people were placed in isolation 1,420 times in the seven months to May of this year – compared to 758 incidents in the 12 months to October 2014.
HIQA says an improvement in the reporting of incidents may account for some of the increase.
But it found that young people were routinely placed in single separation for verbally threatening staff, bad language and smoking when it should only be used as a last resort in response to physical violence, attempts of self harming and significant damage to property.
In one case, a young person was in single separation in their bedroom for four days for lighting up a cigarette in the lounge.
The report also raised concerns about young people absconding, fire safety and staff absenteeism.
