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Is there possibly a recent break-up with a girlfriend, that we're not hearing about?
Earlier in the case, I also was thinking along the lines of possible depression/suicidal, coinciding with the wedding of his brother; especially as it seems that Liam does not currently have a significant other. But I read in the DM article that Eamonn and his fiancee already have children together... so, it isn't a big life-altering event, really. They've apparently been together for a while, so, simply "making it official" wouldn't really affect Liam all that much at this point, I wouldn't think. (?) So, not sure about the wedding-blues dynamic anymore.
Unless he's been lamenting for a long time about not being able to find 'the one" for himself; or possibly a recent break-up.
I'll admit, I've been expecting to hear something from Liam's partner/SO, or an Ex-, but haven't seen anything.
I'm still leaning more toward him having a bit of a dysfunctional, alcohol-infused breakdown -- having been over-stressed with his usual work, and organizing this whole stag-do, plus probably feeling continued pressure, for everything to go smoothly for everyone. Then on top of that, when he fell, he may have hit his head, making cognitive matters even worse.
Poor Liam, seems he took care of everything but himself.