Nearly 700 children have been waiting at least a year for to see a speech and language therapist.
Speech and language therapy provides treatment for people who have difficulties with communication, eating, drinking and swallowing.
According to new figures released under the Freedom of Information Act, more than 11,300 children are waiting for their initial assessment.
Some 247 of them have been waiting at least two years, while 438 have been waiting for longer than 12 months.
Barnardos CEO Suzanne Connolly said the figures are hugely concerning.
“Children can’t wait,” she said. “If they don’t get the service they need at the right time, it will really affect their development and their long-term future.
“It is very important that when there is an identified need for an assessment, it happens as soon as possible so children get the services they need.”
Saoirse Brady, policy director with the Children's Rights Alliance, said vulnerable children are being left untreated at a crucial stage.
“It is concerning in terms of those children’s development,” she said.
“We know that if there is a speech and language issue that is picked up early – before child is five – and they are given treatment, they may not have long-term effects from it.
“It is crucial that we actually ensure they get an initial assessment and get any further treat as quickly as possible.”
Some 381 children have been waiting in Dublin, Kildare and Wicklow for at least a year – with nearly half of these waiting for two years.
Meanwhile, 151 children in Cork and Kerry have been waiting at least a year – including 95 who have been waiting longer than two.
Reporting from Eoghan Murphy