Episode 1 and 2 of Murder... I was convinced IB was guilty. By the 3rd episode I wasn't sure. There wasn't genuine evidence against him, which is why the DPP wouldn't prosecute him. He's an unlikeable, violent man who never shuts up, even when it's obvious he's making himself look bad. However, no matter how IB comes across, there was no evidence to tie him to the murder. Marie Farrell's evidence was a complete lie.
The friend who said a poet was bothering Sophie only (according to the Sky programme) spoke up at the trial and not before. That, to me, could be a false memory - someone desperately wanting justice for their friend and subconsciously half remembering something as it fits with the suspect,. Crucially though, she never named IB.
As much as the murderer could be IB, it could also be many other people who were in West Cork at the time. People are viewing IB through a retrospective lens - knowing all about his violent past, knowing about the murder, knowing he's the chief suspect. It's easier to consider him guilty looking at it that way. However, the Gardai or press didn't go through other individuals lives as forensically. They might have similar dark episodes in their lives that make people consider them guilty.
The footage of IB Christmas Day appears inconsistent with the comments by the woman who made the video. She said he had his hands in his pockets the entire time apart from when, off camera, he shook her hand. That's when she saw the scratches on this hand. Jim Sheridan showed parts of her footage wher IB had his hands out of his pockets.
The Gardai massively destroyed any chance of the murderer being caught. From the state pathologist insisting the body stayed outside overnight until he arrived to the loss of evidence, deciding on the murderer early on and not seemingly considering other suspects to the coaching of Marie Farrell and persuading her to give false evidence to identify IB, both directly and indirectly.
I'm not sure who killed Sophie. As much as it might have been IB, it could be anyone. Because of a badly run investigation we will probably never know. There are 2 guilty parties in this murder - the murderer and the Gardai, as they let Sophie and her family down by their inept investigation.