"Sex tapes" of Patrick Quirke and Mary Lowry and, separately, with his wife Imelda, were discovered by gardai on the killer's computer.
But the three intimate audio recordings were not put before the jury because they were deemed potentially prejudicial since the judge said they suggested a "strange proclivity" of Quirke's.
A garda reconstruction of the discovery of Bobby Ryan's remains in the tank was also ruled out by the judge, on the grounds that the evidence it offered was "frail".
The jury was also prevented from hearing evidence that Quirke had visited a blog with the title: "Why Joe O'Reilly thought he had committed the perfect murder," again on the basis that it could be prejudicial.
The recordings of Quirke's sexual activities were among the evidence found on a computer hard drive taken from his house in a garda search.
However, they were not heard by the jury after Judge Eileen Creedon decided they offered no benefit, since there was "very little dialogue" between the parties involved.
She said she had been persuaded by the defence that the tapes touched on the sexual behaviour of the accused and his "strange proclivity" and therefore might be prejudicial.
The prosecution told the judge they had provided the court with four recordings but the judge said that when she had played the memory stick provided to her, she only heard two. She was told three were of an intimate nature.
Detective Garda Paul Fitzpatrick of the National Cyber Crime Bureau told the court - in the absence of the jury - that he associated the recordings with a Nokia mobile phone.
The defence argued that the computer evidence, including the searches for human decomposition, should be excluded in full as it could not be established that Quirke was operating the computer.
It also sought to have the tape of the chat between Mary Lowry and Flor Cantillon - secretly recorded by Quirke - ruled out, claiming the matter had not been properly investigated, but this was heard by the jury.
The defence described the tapes as "highly prejudicial" and said that they "proved nothing".
Bernard Condon SC said the jury must be instructed that they could draw no inference on how the material ended up on his client's computer, otherwise "we are into the realm of speculation because it has never been investigated".
He argued the recordings had never been put to Pat Quirke in interviews.
Michael Bowman SC, for the prosecution, said: "The tapes described as 'sex tapes' were dealt with on two separate occasions and on each occasion the defendant chose to exercise his rights."
Mr Bowman said the fact that there was a recording between Ms Lowry and Mr Cantillon taken out of Mr Quirke's property would be established by facts.
"We can't say how it came to happen," he said. He added that the court would warn the jury not to speculate on any element of the case.
Mr Bowman said the recording was important as it "clearly communicates" the interest Quirke had regarding Mr Cantillon and Ms Lowry.
Judge Creedon ruled the material in - but said internet searches for Jo Jo Dullard, Siobhan Kearney and Joe O'Reilly could not be put before the jury since these names were part of "notorious cases about which many people could conduct searches and could be prejudicial".
The prosecution had sought to have the two rectangular concrete lids of the run-off tank taken into the courtroom to be shown to the jury.
However, the defence counsel objected strenuously, with Mr Condon describing the exercise as "entirely unnecessary".
"This is a piece of theatre - the doors of this court will open and these enormous slabs will be rolled into court and guards will huff and puff as they bring them in," he said.
David Humphries BL, for the prosecution, objected to Mr Condon's remarks, saying: "This is a matter that can be organised so that there is no huffing and puffing.
"This is not a piece of theatre, not for titillation," he said.
However, Judge Creedon decided that the introduction of the slabs was "not necessary".
Meanwhile, the jury also did not hear Michelle Ryan's claim that her father Bobby had informed her that Patrick Quirke had warned him to "stay away from Mary Lowry" at a Brendan Grace concert they had all attended.
The defence objected on the basis that it was hearsay.