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These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Base Breaker: Series 4. It was assumed that losing Robert Sheehan as Darren would be a radical change for the show, but what wasn't predicted was how much the overall writing would change. While every individual episode was still of a high quality the overall shape of the series was utterly aimless: plot after plot was brought in and then pushed away before anything of consequence came of it. Character after character was implied to be about to do something important and then killed off without consequence before that could happen. This could be forgiven if the new cast of Garda officers were interesting, but they were basically everyone's least favourite part of the show; the heavily-advertised presence of Brian F. O'Byrne proved to be wasted as well on an especially flat character. The only thing that happened in the series that will actually matter in the long run is that Siobhán has decided to betray Nidge. Other than that the whole season gave many the impression it was the first half of a longer story.
Designated Hero: Darren. Over the course of the series, despite saying that he wants to get out of crime, performs more and more criminal acts, even he kills John Boy and it's no longer necessary. Rosie even finally notices this and leaves him for good at the end of the second series.
Genius Bonus: Every character in the show shows a preference for the Glock, except in the opening scene, when Bobby gets shot by a masked man with a Beretta. It's only later in the series, we see Hughie carry one a scene or two before he's revealed as Bobby's killer.
Growing the Beard: Series 1 had its wings clipped by RTE in pre-production, fearing the grimness they were aiming for would drive viewers away. As a result it's poorly paced, padded, occasionally quite goofy (especially at Nidge's wedding) and the characters' personalities were a bit vague. Series 2 corrected these problems and dug deeper into the culture of crime. Series 3 upped its game to the point that pundits were no longer adding the suffix "for an Irish show" to their praise.
The casting improved as the series progressed too, with actors looking less like they walked out of a car insurance commercial.
Many would argue that with Rosie gone, the Romantic Plot Tumor that existed in the first two seasons is no longer restricting the main plot to being about Darren and Rosie's relationship.
Rescued from the Scrappy Heap: Fran, who was a hammy villain in Series 2, became the very bleak comic relief in Series 3 and was much better for it, basically becoming the new Ensemble Darkhorse since Nidge became the main focus of the show.
Win Back the Crowd: Series 5 is being seen as a return to form, especially with the long awaited tension between Nidge and Fran coming to the fore.
Tommy and, by extension, Siobhan, once Tommy gets brain damage. Tommy is reduced to a Man Child who ends up periodically bleeding from his ears as a result of brain damage and Siobhan is driven to being an emotional wreck dealing with Tommy's issues.
Hell, Siobhan in her own right. She gets used by John Boy to carry money through the airport, raped by a high ranking IRA man, left to deal with Tommy's brain damage as mentioned above, puts herself in a position where she is risking her life passing information about the gang onto the Gardaí and pushed to the point where she pushes Pauley off a balcony. She later gets killed by Patrick when he finally takes down Nidge.
Debbie. From her introduction, she's subjected to a Trauma Conga Line, starting with a heroin habit, being beaten by John Boy, forced into prostitution, kicked out of the brothel she's been working in, reduced to stealing money from her own parents, forced to work as a drug mule and finally dying of an overdose.