The backing guitar for this wonderful song mainly involves fingerpicking through a pleasant chord progression rooted in the key of A minor.

The song was originally recorded when Gary Moore was playing with Irish rock group Thin Lizzy. Since then it has become known as a bit of an epic, with many different versions (Gary Moore never plays it exactly the same way twice). The lead is fantastic blues / rock indeed and we'll be having a look at that next. First, lets look at the backing chords...

A standard fingerpicking pattern is used to play each chord. The pattern is T12321. Although some of the higher chords involve using slightly different fingers to pick. There are 6 notes played per chord, and usually each chord is picked through twice (T12321 * 2). The best way to approach the fingerpicking is to 'assign' a finger to a particular string:

Use the Thumb for the root note on the low strings. 1st finger on the G string, 2nd finger for the B string, and 3rd finger for the high E string. There are occasional exceptions, but generally this guideline will see you through the backing chords.

Fingerpick your way through the following chord progression (and remember each chord is picked through twice):

Am
Dm7
Dm7 with G in the Bass You can use your 4th finger on fret 3 of the Low E string instead for the first note (G)of this chord if it gives you problems. That way you can keep the previous Dm7 chord fretted, and just alter the bass note)
C major7
F major
B Ø half dimº
E7
F major

That is the main chord progression. Once you know it the rest will be much easier to get! There are a few other chords used occasionally, and sometimes the backing will start on the Dm7 instead of the Am.

The last main progression for the song is the outro which is repeated until the end. Those chords are:

Am
Dm
Am

Pick these chords using T12321 just once through each, then:

F major (just 3 notes)
Em (just 3 notes)

Repeat until finish!