Graham Dennis (author of the article) absolutely does not ignore this. He regularly points out that Irish rail ridership exploded almost in lockstep with GB rail network growth despite remaining entirely publicly run. Even right at the top of this article he points out that the growth had begun long before privatisation. He goes into a lot more detail in his book.
Possible complicating factor is that at about the same time Irish Rail started experimenting with radical new concepts like having trains that were not from the early 1960s, and having more than one commuter line in the entire country. So possibly hard to read too much into it; there’s an argument that Irish journeys just went up because the system was becoming rapidly less terrible.
(I remember being amazed as a kid in the 90s on encountering a mainline train that had automatic doors. Irish Rail was pretty behind the times for a long time.)
The growth didn't happen long before privatization in the UK though. It turned around almost exactly when the privatization process started (which wasn't an immediate overnight event).
I'd argue that the upswing had started in the 1980s and took a hit along with the economy between black monday (87) and ERM2 crash/black wednesday (92)
The privatization took years however, it wasn't an overnight thing. A lot of the process was just writing down and professionalising a lot of aspects that were previously ad-hoc, so the benefits started before the precise date drawn on the graph. The law was passed in 1993 but preparations began before that, it was clear this was going to happen even in 1991.
