Almost 85 per cent of prisoners left school at or before the legal age of 16, and over 40 per cent before they were 14, according to a leading literacy expert.
Dr Mark Morgan of St Patrick's College, Drumcondra, was speaking at a conference on literacy and the law organised by the National Adult Literacy Agency (NALA) at the weekend.
Dr Morgan, who has carried out a survey of prisoner's literacy, said that almost 60 per cent had experienced some problems with literacy, writing and maths after leaving school.
The survey also showed a strong link between low literacy levels and violent crime and crimes against property.
Mr Matthias Kelly QC, chairman of the Bar of England and Wales, told the conference it was estimated that 60 per cent of people in British prisons suffered from functional illiteracy.
He said that one in five adults in Britain could not use the Yellow Pages to find the page number for plumbers in the alphabetical index.
The situation for numeracy was even worse, he said. One in four could not calculate the change they should get out of £2 if they spent 68p and 45p.
"For people with limited basic skills this means that they are at a serious disadvantage, not only in work but also in encounters with state institutions such as, for example, our courts or legal system," he said.
In Germany only 8 and 7 per cent respectively of the population had these problems. It was up to trial judges to deal with literacy problems sensitively, he said, and he suggested that everyone asked to give an oath in court be given help with it, unless they specifically asked to read it themselves.
He also said that literacy problems should not be a barrier to serving on juries. "How can it be said that the determination as to whether or not someone is lying or not about his or her role in a fight outside a pub on a Saturday night requires these skills?"
Kieron Wood, assistant editor of the Sunday Business Post and a barrister, said that much recent legislation was full of "legal gibberish" and incomprehensible to most people.
"The 1996 Divorce Act, which fundamentally affects the lives of tens of thousands of people, was drafted in terms which make it impossible for anyone to understand - except lawyers of course," he said.
He gave as an example Section 36 (b) of the Act, dealing with probate tax, which refers to the Finance Act, and contains 92 words in a single convoluted sentence.
Ms Inez Bailey of NALA said the agency offered a plain English service to those attempting to communicate with the public, including the editing of documents, offering advice on how to make documents clearer and delivering training on plain English.
The agency also publishes a booklet on plain English, Writing and Design Tips, and has collaborated with the DPP's office on booklets on the role of the office and going to court as a witness.