If you use fingering like a whistle, you'll find some notes of your Irish Flute sound "off." The traditional Irish flute is purposely tuned a bit flat. The F# is flat, the A is sharp, the C# is flat, and the bottom D is flat. Flutists believe these "off" notes came to the flute in the early 19th century. Explanations include: flutes were not originally played with "whistle" fingering; the "off" notes are a result of a 3-octave tempered scale; the "off" notes compensated for the flutist Charles Nicholson's method of blowing the bottom notes (G-D) more strongly than the A; and that the A and C# need to be slightly "off" in order to produce a good cross-fingered C-natural. Regardless of the reason, the traditional tuning for the Irish Flute is a bit flat.
Tuning a Flute with the Slide and Embouchure
Until you can produce good strong notes over the lower and upper octaves, don't worry about tuning. When you are ready, start by positioning the tuning slide so you produce a strong, full G at proper pitch. Beginners usually blow harder on the second octave causing it to sound sharp. With the flute, you don't blow harder to get the second octave; you adjust your embouchure by tipping your head up or down slightly, or by rolling the flute toward or away from you. Blowing down into the hole makes a note flatter, and blowing across the hole makes it sharper. Your tuning will improve, as your lip muscles strengthen and you develop a tighter embouchure.
Tuning a Flute with the Head End Cork
If the head-cork moves, your flute will get out of tune with itself (i.e., low notes not in tune with high notes). If this happens, use a 1/2-5/8" dowel apply steady, gentle pressure to move the cork. Keep the barrel section slid over the tuning slide so that you do not damage the slide. The cork is in the correct position when you are able to play both low B and high B in tune with each other.