Heather Perrin (60) ran a solicitors' firm before being appointed to the bench and was asked by her long- term friend and client Thomas Davis to help draw up a will for him.
Perrin is alleged to have altered the will, without his knowledge, to benefit her son and daughter.
The alleged deception came to light when a new firm of solicitors took over Ms Perrin's practice when she was made a judge.
They thought it strange that so much of Mr Davis's will, which was worth approximately €1m, was left to the Perrin children.
When confronted, Ms Perrin initially claimed it was a mistake by her secretary, but then told gardai that she created the will in line with her client's wishes.
Perrin, with an address at Lambay Court, Malahide, has pleaded not guilty to deceptively inducing Mr Davis to bequeath half of his estate to Sybil and Adam Perrin, with an address on Fairview Strand, on January 22, 2009.
Prosecuting counsel Dominic McGinn told the jury that they will hear from Mr Davis, who is in his early 80s, and was a long-standing client of the accused's law firm.
Mr McGinn said in early 2009 Mr Davis decided to draft a will.
He instructed Ms Perrin that in the event of the death of him and his wife he wanted to leave €2,000 each to Perrin's children as well as sums of money to various churches.
Mr McGinn said the rest of the estate was to be divided up between Mr Davis's nieces.
This included a house in Finglas, €750,000 from the sale of a house in Gorey and a large sum of money on deposit with EBS.
Counsel said there would be evidence that Mr Davis signed the will in Perrin's office but did not read it over because the accused was in a rush and he trusted her.
Several months later he received a copy of the will which complied with his instructions but was unsigned and undated.
Mr McGinn said after the accused was appointed as a judge in February 2009, the firm that took over her practice sent several letters to Mr Davis querying several legal matters.
He said Mr Davis replied with several increasingly irate letters asking the firm not to contact him again and demanding the return of his legal documents and jewellery, which was stored in the firm's safe.
Mr McGinn said that these letters were drafted with the help of Perrin.
The trial continues on Monday before Judge Mary Ellen Ring and a jury of eight men and four women.