In my opinion, I'd say around 2006/2007. There were plenty of characters around like Biggins, Degsy and Flutterin Bantam...the type of posts you'd stop scrolling to read either for the laugh or a brutal insight. They have since been banned and replaced with bland, boring, predictable PC posters like The Backwards Man, who can be relied upon to never upset the transexual Muslim cartel that run the place now. The "Thanks" button wasn't a thing yet so there were less bootlickers following mods around trying to get noticed. The mods weren't as heavy handed back then either. Not sure who ruled AH back in '06 but things turned to shit when Dr Bollocko the unapologetic Leftist degenerate came to power. Things went from bad to worse when shrill, hysterical Whoopsydaisydoodles became a mod.
If I had to pinpoint the beginning of the end for Boards, I'd guess it was in 2008 in the run up to the US Presidential election. Here we had a bunch of Irish bandwagon jumpers (who had previously never showed an interest in Irish politics or even voted in their local elections) pledging support for Obama in attempt to prove how un-racist they were and virtue signal their progressive credentials to their peers. This opened up the floodgates for imported American university Leftist trash like SJWs, white privilege, cis-scum, Black Lives Matter etc. None of these narratives are relevant to Ireland or Irish people yet they've poisoned the mind of our youth thanks to sites like Boards pumping out this propaganda and silencing dissent.