I believe it is as simple as this and not as complicated as it seemed last night, because the HL show revealed the initial Noirin/Siavash talk.
Siavash told Noirin in confidence that he believes many people in the house think she's a bunny. He gave an example, after being pushed, of Kris/Karly telling eachother about it (he means he overheard).
Noirin to Siavash: 'I promise to you, I won't say anything.' - LOL 
Noirin totally betrayed this by then bringing it up with Lisa's team, rather than trying to keep her mouth shut. When Siavash came in to try and resolve it, she 'fed him to the wolves', proverbially.
Basically, Lisa's crew started demanding names/details from Noirin, then both Lisa's crew and Noirin started demanding names/details from Siavash.
Noirin wanted to minimize her damage by trying to make Siavash spill all the beans. Siavash was reluctant and tried to dodge this, because:
1) He said it in privacy to Noirin, so why should he?
2) By doing so, he is digging his own grave, so to speak.
3) Noirin should say it if anything, she's the privacy-breaker.
Eventually it got out that Kris/Karly were involved.
Throughout this, Siavash was left to try and save himself, a little lying if needed, because he had been royally done by Noirin.
That's how I see it. Of course this assumes that Siavash was telling the truth about overhearing Kris/Karly, which the forum says is true; at the bus stop. So I'll assume that. As for the rest of them, it's just what he believes, fine.
To Note: Charlie realised Siavash's discomfort and how he had been played, and told Noirin to spill everything rather than Siavash, because she was the confidentiality breaker. Well done to him.
To Note: Freddie was lying or forgot, HE WAS THERE when the names were mentioned. He was sat in the circle of four. Maybe it was a white lie, because the convo was confidential and he needed to save himself.
I see Noirin as the confidentiality wrecker and Siavash (+ Freddie a little by the end) as the scramblers, who tried to dodge and lie a little, to save themselves. As most would.