Heather Perrin (60) is accused of attempting to deceive Thomas Davis into bequeathing a large portion of his estate, which was worth about €1m, to her two children while she operated a solicitor's practice.
Mr Davis, who is in his 80s, spent the day in the witness box giving evidence of the alleged attempt to alter his will without his knowledge.
He described a rushed meeting with Perrin shortly before she was officially appointed a judge in which he signed a will but was not given a chance to review it.
Perrin, of Lambay Court, Malahide, has pleaded not guilty at Dublin Circuit Criminal Court to deceptively inducing Mr Davis to bequeath half of his estate to Sybil and Adam Perrin at her office on Fairview Strand on January 22, 2009.
Mr Davis told prosecutor Dominic McGinn that he knew Perrin for many years because his wife was heavily involved in the Girls Brigade with her. He said he also knew her two children since they were young.
In January 2009 he gave instructions to Perrin concerning a new will and went to her office on January 22 to sign the document.
The prosecution presented Mr Davis's will which bequeathed €2,000 each to various churches and €2,000 each to Adam and Sybil Perrin if Mr Davis and his wife passed away.
It also ordered his house be sold and the proceeds divided up between his two nieces and the rest of his estate to be divided equally between his two nieces and the Perrin children.
Mr Davis identified his own signature on the document as well as those of two witnesses. These were the accused's husband Albert Perrin and her secretary, Pauline Ball.
The witness said it was a short meeting because Perrin had other urgent work to do. He said he did not get to read the document, nor was it read over to him. He said he never gave instructions to leave half his residual estate to Perrin's children and that he wouldn't have knowingly signed any document which did so.
He said he was not given a copy of the document until several weeks later when one was posted to him. He said his document corresponded with his original wishes to divide his residual estate between his two nieces only and he was happy with it.
Mr Davis said later that year he began receiving letters from the firm that took over Perrin's practice. He said Perrin wrote responses to these letters demanding the firm stop contacting them and return all their wills and other legal documents.
He said he read and signed these letters before they were sent and added: "We didn't know any better." He said the accused would then take these letters and send them.
"We trusted Heather Perrin and believed her," he said.
He said that when he received a letter from the law firm querying his will and how much he had bequeathed the Perrin children he went down to their offices with his friend and his niece.
He said he couldn't believe it when they showed him the will he signed the previous January. He said he made a new will that day and made his nieces the executor of his estate instead of Ms Perrin. He continued to bequeath €2,000 each to her children.
Under cross-examination from Patrick Gageby, for the defence, the witness agreed he forgot things sometimes. At one stage during his evidence he failed in his initial attempt to identify himself in a photograph taken at a birthday party of Perrin's child.
The trial continues.