Greetings to a fellow Aussie!
More experienced forum members will probably elaborate on this, but here are a few quick responses:
The “Irish” flute generally refers to a wooden simple-system flute of the type that was used in classical western orchestras up to mid-19th century, when it was gradually replaced by the Boehm metal flute that your friend plays. (This is why flutes are classified as “woodwinds” today - a bit like the “metal woods” that golfers use).
The classical wooden flutes had 6 open holes to play a D major scale over 2-3 octaves, plus a system of normally-closed holes opened by pressing keys to enable playing of notes outside the D scale. Most Irish music is in D or G (or modes thereof) and can be played without using the keys. The wooden flute also has a conical bore to enable the maker to tune the octaves properly.
The modern Boehm flute has a cylindrical bore with a conical head to do the same thing as regards tuning. It has a system of normally-open keys, with no open holes. It produces a brighter, silvery tone and is easier to play across 3-4 octaves, in every key.
The embouchure holes on “Irish” flutes are generally smaller, but there are many variations favoured by different makers and players. There is a key differenec in the way the player shapes the blowing embouchure and directs air - broadly, the “Irish” style directs air down into the hole more than the modern Boehm style, which is more across the top of the hole - the Irish sound is more nasal, “harder”, perhaps rougher.
There is a fundamental difference when it comes to ornamentation. In western classical music, “ornaments” generally consist of “grace notes” which, although short in duration, are intended to be heard as audible pitches. In Irish music, the ornaments are more properly “articulations” - for example, a “cut” is not meant to sound like a very short high note placed just before a melody note, even though it is played that way.
When it comes to the music, I can’t hope to even scratch the surface here. Suffice to say, the music is structured around several standard dance tune forms (jigs, reels, Kerry polkas, hornpipes) and there are literally thousands of tunes of each type. An important point is that the music was traditionally handed down by ear. Classical musicians are taught to regard the written score as THE music, and to adhere closely to the score. Traditional musicians are expected to make the music their own once they have learned a tune - including variations, personal style and subtle factors.
By the way, there are several good performers of ITM (Irish Traditional Music) who play the Boehm flute.
If your friend wants to try a wooden simple system flute, I can recommend the Casey Burns Folk Flute (I have one). Inexpensive and high quality.
Terry McGee’s website has a load of good info on the history of different flute types and structures.
