I don't have any charts or diagrams to offer, but if you have found some for the 20 button C/g concertina, you can use those same diagrams, but the chords will sound one full step higher.
The most obvious example will be that any combination of buttons in that outer row will be a D major chord on the push, instead of C major chord. For example 3 4 5 on the push will be D F# A.
Less obvious will be that those same buttons 3 4 5 on the pull will play E G B which is an E minor chord, instead of D minor.
A very useful pair of low chords will be a partial D major - buttons 1 2 3 on the push - D A D (instead of C G C) and A major 1 2 3 on the pull - A C# E (instead of G B D)
You will find that the inner row will be A major chords on the push anywhere of course, but it is nice to have that reversal, and to have a lower range option.
The third most common chord in the key of D would be a G major chord, and in the left hand that would be 4 5 14 on the pull for G B D (instead of F A C) where 14 indicates the 4th button on the inner row.
This shift does mean that which notes and chords are available to you will be different than the list available on a C/g. For example you won't have a C natural. But you have the C# which is so important in the D major scale, and that is very useful in Irish music. (the C# will be found on your instrument wherever the C/g chart indicates a B )
Some full chords won't be available on a 20 button instrument, either because one note isn't there at all, or because one note is not available in the same direction. But usually 2 notes from the chord are enough, and this is often a better choice than playing the full chord to accompany singing or a melody instrument anyway.
Edited by Tradewinds Ted, 24 March 2017 - 08:47 AM.