As Michael pointed out, it has a lot to do with DADGAD tuning. The High Kings play a more or less constant D-A drone on the guitar, to allude to Irish bagpipes (called "uilleann pipes"; DADGBE allows the same effect without too much relearning). If you choose to subtract that, Dmaj7 becomes F#m, and D6 becomes Bm, etc. Here is what I heard, formatted by my songbook software:

 
At 

D  
Boola

D^maj7 
vogue, as the 

D⁶ 
sun was setting

 
O'er the 

D^sus24 
bright May 

D  
meadows of 

D^sus24 
Shelma

D⁶ 
lier

 
A 

D  
rebel 

D^maj7 
hand set the 

D⁶ 
heather blazing

 
And 

D  
brought the 

D^maj7 
neighbors from 

D⁶ 
far and near

 
Then 

D  
Father 

D^maj7 
Murphy, from 

G/D  
old Kil

D⁶ 
cormack

 
Spurred 

D^sus24 
up the 

D  
rock with a 

D^sus24 
warning 

D⁶ 
cry

 
"Arm! 

D  
Arm!", he 

D^maj7 
cried, "for I've 

D⁶ 
come to lead you

 
For 

D^sus24 
Ireland's 

D  
freedom we'll 

D^sus24 
fight or 

D  
die"