I have seen two versions, one in the key of G, and one in C -below.
C C7 F F6
Oh Danny boy, the pipes, the pipes are calling
C Am Dm G7
From glen to glen and down the mountain side
C C7 F Fm
The summer's gone and all the flowers are dying
[the above line ends "all the roses dying" in Elvis' version]
C G7 C C7
'Tis you, 'tis you must go and I must bide
C F C G7
But come ye back when summer's in the meadow
C F Dm G7
Or when the valley's hushed and white with snow
C C7 F Am7 F
And I'll be here in sunshine or in shadow
C Dm G C
Oh Danny boy, Oh Danny boy, I love you so.