Here is an example of the almost-mechanical arrangement process I had in mind. Starting with a tune picked more or less at random (one I posted here in another thread) add voice labels to it:

X:1
T:Gleanntain Ghlas Ghaoth Dobhair
B:Eamonn Jordan, Whistle and Sing
M:3/4
L:1/4
Q:1/4=100
K:DMix
[V:1] D   |A> B c|A2  D   |A2  A|A F D   |C B, C|D2  D|D3-|D2
[V:1] A/B/|c A  G|A2  B/c/|d2  c|A2  d/d/|c B  A|G D E|C3-|C2
[V:1] c/c/|c A  G|A B c   |d2  c|A2  d   |c B  A|G D E|C3-|C2
[V:1] D/D/|A B  c|A2  D   |A F A|G F D   |C B, C|D2  D|D3-|D2|]

Transpose it up so it's in a good range for D whistle (highest note A) and also transpose it down to get another part a sixth lower. Stick the two parts together to get a brainless instant chorale:

X:2
T:Gleanntain Ghlas Ghaoth Dobhair
B:Eamonn Jordan, Whistle and Sing
M:3/4
L:1/4
Q:1/4=100
K:Amix
[V:1] A   |e> f g|e2  A   |e2  e|e c A   |G F G|A2  A|A3-|A2
[V:2] C   |G> A B|G2  C   |G2  G|G E C   |B, A, B,|C2  C|C3-|C2
%
[V:1] e/f/|g e  d|e2  f/g/|a2  g|e2  a/a/|g f  e|d A B|G3-|G2
[V:2] G/A/|B G  F|G2  A/B/|c2  B|G2  c/c/|B A  G|F C D|B,3-|B,2
%
[V:1] g/g/|g e  d|e f g   |a2  g|e2  a   |g f  e|d A B|G3-|G2
[V:2] B/B/|B G  F|G A B   |c2  B|G2  c   |B A  G|F C D|B,3-|B,2
%
[V:1] A/A/|e f  g|e2  A   |e c e|d c A   |G F G|A2  A|A3-|A2|]
[V:2] C/C/|G A  B|G2  C   |G E G|F E C   |B, A, B,|C2  C|C3-|C2|

Sounds basically okay, but the lower part's out of range on the whistle. So lift a few notes up, taking the alternatives from within the same chord we have at each moment (the process has produced a four-chord number, the chords are A, D, G and E minor):

X:3
T:Gleanntain Ghlas Ghaoth Dobhair
B:Eamonn Jordan, Whistle and Sing
M:3/4
L:1/4
Q:1/4=100
K:Amix
[V:1] A   |e> f g|e2  A   |e2  e|e c A   |G F G|A2  A|A3-|A2
[V:2] A   |G> A B|G2  E   |G2  G|G E E   |D D D|E2  E|E3-|E2
%
[V:1] e/f/|g e  d|e2  f/g/|a2  g|e2  a/a/|g f e|d A B|G3-|G2
[V:2] G/A/|B G  F|G2  A/B/|c2  B|G2  c/c/|B A G|F E D|D3-|D2
%
[V:1] g/g/|g e  d|e f g   |a2  g|e2  a   |g f e|d A B|G3-|G2
[V:2] B/B/|B G  F|G A B   |c2  B|G2  c   |B A G|F E D|D3-|D2
%
[V:1] A/A/|e f  g|e2  A   |e c e|d c A   |G F G|A2  A|A3-|A2|
[V:2] E   |G A  B|G2  E   |G E G|F E D   |D D D|E2  E|E3-|E2|]

Done. You could make the rhythm more varied between the parts if you want.