The bass goes as follows: 

        Am7                     Dm7                     Em7
E |------------------|-----------------|-----------------|
B |------------------|-----------------|-----------------|
G |------------------|-----------------|-----------------|
D |------------------|--4-4-5-5-5-5-7-7|--6-6-7-7-7-7-9-9|
A |--4-4-5-5-5-5-7-7-|-5---------------|7----------------|
E |-5----------------|-----------------|-----------------|

Any variation is welcome. Listen to Gary Moore on Blues for Greeny for inspiration.
His cover is pure perfection!


Verse 1 (Am  Dm  Am  x 2 / Em  Dm  Am)

Am
I've got a long grey mare, she won't let me ride
      Dm
I got a long grey mare, she won't let me ride
      Em
She makes me brush her in the morning
    Dm                     Am
And put her to bed every night

Verse 2 (Am  Dm  Am  x 2 / Em  Dm  Am)

Long grey mare, why can't we be just like before
Long grey mare, why can't we be just like before
We used to ride so hard
Until we just couldn't ride no more

Break:

Verse 3 (Am  Dm  Am  x 2 / Em  Dm  Am)

Look at me grey mare, don't I feed you everyday
Look at me grey mare, don't I feed you everyday
But when I dig you with my spurs
You turn around the other way