@JaneJeffer
I think Keenan might be a red herring and it was his wife who chained him to the radiator and fucked off and left him. But then there was a calling card so I don't know.
I've had a quiet day working from home so plenty of time to ponder. I think Keenan might also be a red herring, or at least an unwitting part of what my theory is now. Particularly after reading the article
@DuncinToffee linked to.
My theory is now that Adam Corry was an ill old man and perhaps realised he didn't have long left. We know that he used a fake name to phone the local fisherman 4 months ago, to find out more about the island the bodies were found on. I'm wondering if he got together with the relatives of the other victims to try and stir things up to get the police looking again and hopefully get some closure before he died. This would also explain Tory taking an interest in Izzy (to get close to another family member of a "potential" victim, particularly someone who is close to the police investigation) as well as Tory's recent return to Belfast. Not sure how the Harkins fit into this, other than she would be the one with the connections to kidnap Keenan and plant the calling card at the scene of the crime. All of this is the reason that the police are now looking into it again. The police also found the bodies because of information Adam Corry gave them while the police were following up on the kidnapping of Keenan. So Adam's plan worked, it got the police looking into Goliath again and made them find the bodies of the 3 male victims, including his brother. I'm not sure if Adam knew who Goliath was, the bits of his diary we saw show that he had theories rather than conclusions. But I think he definitely suspected police involvement / cover up. Adam just wanted to know what had happened to his brother before he died.
I'm not sure about the Corry / Emma affair. Adam knew his brother was having an affair with a married woman but he doesn't know it was Emma, he had guessed that and he may not have guessed correctly. I wonder if the pendant is a message, a warning from Goliath to Tom. Perhaps Tom thought Adam was getting too close to the truth and had to silence him. I'm not quite sure what I think the truth about Goliath is yet though.
Somehow Twomey found out about the kidnapping plot (due to some connection to Siobhan Harkin - perhaps she's an informer or perhaps he's somehow involved in the organised crime). Twomey is against Tom looking into Goliath again but that's not to say Twomey isn't looking into it behind the scenes, he actually moved himself to keep an eye on Tom / the Goliath Investigation and that move must have been sanctioned at a high level. So perhaps Emma was Goliath and Tom collaborated or Emma found out who Goliath was and had to disappear because she would have been killed if Goliath found out she knew who Goliath was. Tom has been protecting Emma all these years and perhaps he shot Adam Corry because he thought what Adam had found out was a threat to Emma.
I'm not sure why Keenan sent a man to kill Siobhan Harkin, I find it difficult to believe that it's over a debt from 30 years ago - why has he not had her killed in the last 30 years, why now? Maybe he somehow found out she was involved in his kidnapping, or perhaps he found out she is a police informer (if she is) - so a red herring. The other Keenan red herring is possibly the kidnapping of Izzy - Tom had threatened to expose Keenan to his wife, so Keenan might have simply been trying to get him back.
Whoever Goliath was, he/ she wanted to derail the peace process. I'm wondering if Goliath was / is even one person, maybe Goliath is / was a collection of people who had a vested interest in the peace process failing. I find it interesting that the victims of Goliath were from both sides, perhaps Goliath is a name for a collection of gangsters from both sides who didn't agree with the peace process as it would damage their drugs/arms businesses. Perhaps Goliath is / was a collection of senior political figures from both sides who were against the peace process. Maybe Emma is coming back because she thinks it's time for the truth to come out (if she was involved).