I’ve not seen Mary Bergin’s book, but I have quite a few “how to play” books for uilleann pipes, whistle, and Irish flute and nearly all of them have things incorrectly notated, or incorrectly explained, or both. (“Correct” meaning how the person who wrote the book actually plays.)
Usually it’s because the traditional players who write these are not familiar with the norms of music notation. Oftentimes it’s because the players are not aware of how they themselves are playing things (which one sees at workshops all the time, with these people in the flesh).
I’m not talking nuances so subtle that notation cannot express them; of course notation will always fall short of the actual sound! I’m talking about the sorts of things that occur in Irish performance which are an ordinary everyday expected part of musical notation in ‘legit’ (non-trad) music, things that written notation expresses all the time, but which are curiously absent from any notation of Irish music.
So if Mary Bergin has her exact articulation (or lack thereof) correctly notated, good for her! There’s a real need for correctly notated Irish music: people coming to trad music from the “legit” world will be able to plainly see what trad players are doing because it’s written into the music. Trad players oftentimes like to make these things sound mysterious.
The only time I can recall seeing anyone accurately notate a performance of Irish traditional wind music was at a whistle workshop many years ago. I don’t remember who the teacher was, but he had written out, in painstaking detail, several Mary Bergin medleys from her Feadoga Stain album. Every tongue, slur, and breath was notated, just as might be found in a fully notated piece of orchestral flute music. It was an eye-opener for me and extremely useful to have Mary’s approach to articulation clearly laid out for all to see.
Anyhow, from the title of this thread I thought it was going to be about Mary advocating that beginning whistlers not tongue at all, at first. I see it isn’t. But it would make sense, because people coming from Baroque flute, Boehm flute, and recorder always misunderstand the tonguing in trad Irish whistleplaying, that it is carefully chosen punctuation/articulation laid over an essentially flowing legato groundwork (rather than a tongued attack being an essential everpresent aspect of each and every note).