The 60-year-old is alleged to have raped his wife at their family home in Leinster in July 2018.
He has pleaded not guilty to the sole count.
A jury of eight men and four women was empanelled for the trial on Monday, but the trial began yesterday after a delay caused by legal matters.
The case before Justice Karen O'Connor is scheduled to continue for about five days.
Patrick Gageby SC, prosecuting, told the jury the defendant was a garda who retired from the force some years ago.
He had married the complainant in 2007, and the couple have two children together and lived together.
Mr Gageby said the jury would likely hear evidence of a volatile marriage with "ups and downs".
He told jurors they "are not being asked to decide would you have granted a divorce on this or that ground" and that the sole focus for them was whether the prosecution had established that an act of rape had occurred.
He said by the summer of 2018 the couple were sleeping in separate bedrooms in the family home and "not having the sexual intimacy that might otherwise be expected or common from a couple".
He said at this stage the complainant's solicitor was sending letters to the defendant suggesting they divorce and asking him to engage in legal proceedings.
Mr Gageby said the expected evidence will be that on the date of the alleged offence the complainant was in her bedroom and the defendant came in and "announced he wanted to have sex".
Counsel said the woman will testify that she said no, the defendant then lay on top of her and had sex with her at a time when she was clearly saying she was not consenting to sexual activity.
He said the complainant will testify that a week later she confronted her husband and "he said a rather interesting thing".
Mr Gageby told the jury that the man allegedly said "something is coming and you won't know what's coming".
Some weeks later the woman received a court summons indicating her husband had taken a safety and barring order against her, alleging that he was in great fear of her and seeking the court's protection.
"Things did not improve after that and she attended at a garda station and made statements," Mr Gageby said.
He said the defendant was then arrested and interviewed.
The trial continues.