Some children under 16 could be forced to wait up to five years before receiving orthodontic treatment.
The Irish Dental Union claimed it is a shocking indictment of the healthcare system that youngsters and adult special needs patients are being denied essential treatment.
Chief executive Fintan Hourihan told the union's annual seminar for HSE dentists in Athlone that children in north Dublin specifically are on long waiting lists before receiving treatment to straighten their teeth.
Other shortfalls included special needs patients in Cork in need of general anaesthesia having to wait three years for procedures while children in Sligo only wait six months.
"These are just a selection of the many ways in which children up to 16 and special needs patients are being denied essential, preventive dental care," he said.
"Regrettably, all the evidence shows that failure to screen dental problems and arrange treatments at an early stage will inevitably cause significant deterioration in the dental health of such patients."
Mr Hourihan called for an audit of services to identify critical shortfalls in the system which should provide dental health screening and preventative treatments to all children under 16.
He put these shortfalls down to cutbacks and added: "These two vulnerable groups are losing out due to the refusal to replace dentists, dental nurses and other key support staff.
"Also, dental staff are now being asked to care for adult medical card holders who are being denied dental treatments due to swingeing cutbacks in the medical card scheme which is really only managing to offer limited emergency care at present.
"Denying our children and special needs patients proper dental care is a shocking indictment of our healthcare system."