A man woke in his bed in a hotel to find his girlfriend having sex with another man, he told a jury.
"There is no words to describe the shock. The most unbelievable shock I have ever had in my life," the witness said.
The 27-year-old accused has pleaded not guilty at the Central Criminal Court to raping the woman at a Tipperary hotel after a wedding function on July 7, 2008
The witness later told defence counsel Mr Michael Delaney, under cross-examination: "This whole vista is etched into my memory and will be etched into my memory as long as I live."
The man told Eanna Mulloy, prosecuting, that immediately after the incident his girlfriend was "curled up with her knees pushed into her chest, rocking back and forth and wailing hysterically".
He sat down on the bed, got dressed and said "I am going to kill him." He left the room and saw the accused being held by two staff members from the hotel wearing a white towel. He returned to his room after one of the staff members said he was going to call the gardai.
The witness said that he had gone to bed that night after the wedding and left his girlfriend talking to another woman on the stairs.
He said he woke up some time later after "sensing" some movement to his left and said it did "not feel right". He said he was "groggy" and what he saw was "just beyond belief".
He just started to move and propped himself up on his left elbow and his girlfriend opened her eyes. The witness told Mr Delaney that the first thing he said was "what the f***!" before he struck the accused. He said his girlfriend almost simultaneously pushed the man off her.
He accepted a suggestion from counsel that he could have hit the accused a fraction of a second before his girlfriend pushed him away.
He then jumped off the bed and struck the accused again, "vaulting" over his girlfriend's legs who had been lying on her side in the foetal position.
The witness told Mr Delaney that the accused was naked and said his girlfriend appeared to be wearing no clothes, that he could see, apart from her bra.
He agreed that he gave the accused a "good beating". "I tried as best as I could to inflict as much damage as I could," the witness told the jury.
He denied that he had not woken up to find the accused having sex with his girlfriend but that the man was instead lying on his side asleep.
"That is completely and utterly incorrect," the accused replied and denied a further suggestion that it was on this basis, having found the accused asleep in the bed, that he "flipped" and attacked the man.
The man accepted that he had a lot to drink the night and had only had two hours of sleep before he woke up.
He agreed that he had no recollection of being out of bed when his girlfriend came into the room. The trial continues.