This tune is in the key of D dorian. Therefore the available chords are D minor, E minor, F major, G major, A minor, B diminished and C major.
In the A part there are no sections which sound strongly like they need anything other than chord I. Consequently the whole part can be backed with chord I, with chord V in bars four and eight, resolving to chord I to mark the end of the section.
In the B part, the second bar contains the notes “C – G C E C G C”. As this is clearly outlining a C chord (chord VII), I have put one in.
Although it is customary to play chord V throughout the fourth bar in an Irish tune, this particular one sounds better with chord I for the second half of the bar. This is because the notes are “C B C D E D D -”. You can hear that the second half of the bar is very clearly finishing on a strong D, therefore I like to put a D minor chord here to match.
In this version to break up the huge chunk of D minor I have replaced the second bar with chord VII. In a chord I section you can more or less play whatever you want, and I like chord VII here as the second half of the bar contains a C note on a dominant beat, so sounds quite chord VII-y .
In bar seven I create a link from C down to A minor by using G/B, a G chord in the first inversion. This is a very common trick to link any chord to its related minor- play the first inversion of the chord below the related minor chord to create a nice descending bassline. For example, you could play D – A/C# – Bm or G – D/F# – Em. In this particular context G/B works well because it’s in a chord I section and you can always play chord IV (G) in a chord I section, so an inversion of said chord is also perfectly fine.
In bar three of the B part I have replaced chord I with chord IV. You can always replace chord I with chord IV in a chord I section.
I have re-used my link from C down to its related minor A minor as in the A part.
In this version I have replaced all chords with tetrads. The tetrads available in D dorian are the same as that in C ionian, except that now we begin on D minor7. The complete list is: D minor 7, E minor 7, F major 7, G (dominant) 7, A minor 7, B ½ diminished (we will avoid the diminished chord by playing the first inversion of chord V, aka G/B).
In bars three and seven, I have used B♭ major 7 as a substitute for D minor. This chord is borrowed from the mode of D aeolian. The dorian and aeolian modes are only differentiated from one another by the sixth note of the scale (which is flattened by a semitone in the aeolian mode) and Irish tunes in minor keys rarely feature the sixth note in their melodies. This means that if we wish to, we can “borrow” chords from the aeolian mode. B♭ major seven contains the notes B♭, D, F and A. The upper three notes form a D minor triad, so so long as the section in question doesn’t have a B♮ in the melody this will make a good dark substitute for a D minor chord.
In the B part I have used the standard trick of linking a minor chord to its related major using the first inversion of the chord whose root is one below the starting chord. In this case I am linking D minor up to F major, so I use C major (chord VII) in its first inversion, aka C/E.
You will notice that in bar four I have committed the ultimate folky sacriledge and missed out the chord V bar! You can sometimes get away with playing chord IV instead of chord V in the dorian mode- it can give a cool optimistic ending to a phrase of a tune. Just bear in mind that you HAVE to go V-I at the end of a section to make the section sound finished. In other words it is sometimes OK to play chord IV in the 4th, 12th, 20th or 28th bar of a tune in a dorian key, but not in the 8th, 16th, 24th or 32nd.
In the fifth bar I have borrowed B♭ major 7 from the aeolian mode again, which is fine as there is no B or B♭ note in the melody anywhere in this bar. I have followed this chord with Am7, even though it is not a chord V section, because it provides a nice conjunct set of chords dancing around the chord V and coming to rest on it in order to resolve back to chord I (the root notes being B♭ – A – G – A – D).
A part jazzy chords
B part jazzy chords