(in the questions that follow, I am referring to fully chromatic flutes)
With the modern Irish flute, do I need to “lip up” or “lip down” certain notes to bring it in tune or that only happens with the ‘old’ Irish flutes?
With the modern Irish flute do we need to vent some notes to get them to sound correctly (more in tune or without veiling)? Like for example (please correct me if I am wrong):
C# is flat, and needs to be lipped up or vented with the C key, as you say. On an 8-key flute, you can alternatively use the harmonic of the low C# and, possibly vent it with L2. It might be more or less useful depending on the flute.
I’ve been playing some Quebec trad tunes lately (often in A major) and C#'s feature much more prominently than in Irish trad, where it’s usually just a passing note, so if you just blow it without doing anything you will get noticed 
. This forces me to work on the tuning…
E on modern flutes is usually in tune, but it’s the weakest note. Venting it will make it stronger but sharp, so you’d have to lip it down to compensate.
F# is strong and in tune, and it doesn’t need to be vented like it does on antique flutes.
Hope this helps.
I am still a bit concerned about the intonation of the Irish flute…
What can you comment from the information that follows and shed some light?
https://www.irishfluteguide.info/choosing-a-simple-system-flute/
“If you use the traditional Irish flute fingering, like that of a whistle, you’ll find that on most English flutes (Pratten, Rudall & Rose, Nicholson, etc.) and their modern replicas, the F# is flat, the A is sharp, and the C# is flat.
The bottom D on Rudall and Rose flutes is also quite flat.
You will have to adjust your blowing angle across the embouchure hole, or use the metal keys in the original simple-system fingering (see below), to make these notes play in tune.
You can blow downward into the hole to make a note flatter, and blow across the hole to make it sharper; this can be done by tipping your head up or down slightly, by rolling the flute toward or away from you, or a bit of both.
The bottom D on a Rudall-style flute can be played in tune by tightening your embouchure, blowing harder, and by using a flatter blowing angle than you would use on a Pratten.
Some flute makers believe the off notes are casualties of the tempered scale devised to make the flute playable in three octaves.
(Remember that these flutes were originally designed to play classical music).
Other makers, such as Patrick Olwell, believe these toneholes were deliberately placed they way they were to accommodate Charles Nicholson’s idiosyncratic way of playing the flute, in which he blew much more strongly on the bottom notes (G down to D) than on the A.
(Nicholson was a very influential flute player in the early 1800s, who had a hand in designing the large-hole flutes favored by most Irish musicians today.)
Some makers point out that the A and C# need to be slightly off in order to produce a good cross-fingered C natural.
Nineteenth century English flutes were not actually designed to be played with the “whistle” fingering, which is another reason why some of those notes sound off-pitch: if you look at the back of Hammy Hamilton’s The Irish Flute Player’s Handbook (or in The Flute, by Rockstro), you’ll see a chart that shows how the simple-system flute was originally meant to be fingered.
Opening either of the two F-natural keys while you play F# will bring the note closer to pitch; combining this with opening the D# key may bring it even closer.
Opening the long C-natural key while playing C# (all holes open) will bring that note up to pitch as well.
But this doesn’t help you if you have an unkeyed flute: you need to learn how to blow those notes into tune, which is what most Irish players do rather than use the keys to bring notes into pitch.”
I think that article focuses more on the past (the 19th century) than the present (the modern Irish flute), RPereira. And doesn’t go far enough into how things changed over that period. Certainly flutes of Nicholson’s period were diabolical in tuning. At any pitch! They slowly got better in the Rudall Rose days, but still had their issues. And by the time we got to the Pratten’s Perfected era, they were making good progress working those issues out.
When we come up to date, we find a few makers still making what we could call “faithful” copies of the period flutes, and so yes, if you have such a flute, you can expect to have to learn some tricks to accommodate those deviations. But I’m imagining that most makers are striving to make flutes that can play in good pitch without the player having to sell their soul to the Devil to be able to play it.
But that’s not to say it’s easy. We can relatively easily deal with the overall scaling issues. The period flutes had to deal with the pitch of the period making its way north from around 420/430Hz to around 452/455Hz. Stupidly, the same flutes were expected to be OK at both ends!
We can see the effect of pitch by looking at the Scale Length of flutes over the period. I’ve made an attempt at that at this page: https://www.mcgee-flutes.com/CsharpEb.htm . You’ll see an early Clementi Nicholson kicks off the list at 264mm between the top finger hole and the Eb key hole, whereas a late Boosey Prattens comes in at 245mm. I can’t play the Nicholson acceptably at modern pitch, but the Pratten is easy.
Our job is easier since we’re only aiming at one pitch, 440Hz. But we face other challenges too, predominantly in the matter of human reach. Ideally our finger holes would be distributed along the lower half of the flute according to how many semitones lie between them. F# to G is one semitone, so holes 4 and 5 should be about half the distance of holes 5 and 6, which cover 2 semitones. And we shouldn’t see such a big gap as between holes 3 and 4 which is still only 2 semitones. But we can largely deal with that by changing the size of holes, and undercutting the holes upwards or downwards to minimise errors between the octaves. The 6-hole flute can’t be as well tuned as the Boehm flute (which gets around the issues by putting the holes where they should be, and using linkages to make them accessible), but it can be very adequate for practical purposes.
We have to be very careful with anything Rockstro reports as he had his own agendas which render a lot of his writing unreliable. Needing to open the F key to humour the F# note is possibly one of these, as I point out in https://www.mcgee-flutes.com/RockstroAnalysesRRflute.htm . Rockstro was trying to push an early Rudall & Rose up to Philharmonic pitch, which wouldn’t have been invented at the time that flute was made. But it didn’t suit his agenda to admit that. I make it always a habit to distrust Rockstro unless I can find another authority in agreement!
On the F key to sharpen the F# note, I find on my flute (my “Rudall Perfected” model) I can’t hear or see on the tuner any difference in pitch when I open the F keys. It might make more of a difference on a very small-holed flute.
All of the above being a long-winded way of saying you shouldn’t need to vent any keys to be able to play in good tune if you have a well-tuned modern flute.
I am still a bit concerned about the intonation of the Irish flute…
No need to be concerned. ALL simple-system flutes require SOME lip adjustment for intonation or for tone quality. This is normal, and something that baroque flute players work assiduously to perfect. You will get very good at this (or die trying), but it may take a few years; sorry.
SOME flutes have intonation that is more difficult, and some flutes were designed for a different tuning, typically lower in the first quarter of the 19th C, and typically higher in the last quarter.
19th C flutes have been designed to use 19th C fingering, which enables three octaves from low C up to high G (or more for better players than I am). Most 19th C flutes have a “best-ish” tuning, but can be tuned up or down 5 - 10 Hz by moving the slide in or out.
I have three R&R flutes that play nicely in A440: a large, medium and small bear. Two of them are A440 with the slide at 1cm, one with the slide half or less. The medium bear is “just right”, and plays ever so sweetly and easily, now that my lips are decent.