Lenition or Aspiration.                 The following changes occur:
 
b         --  v  bw -- w
c         --  ch
çh       --  h 
d, dh   --  gh
g       --  gh             (2)
j          --  y
k         --  ch 
m        --  v  mw -- w
p         --  ph 
qu      --  wh
s, sh   --  h             (1)
t, th    --  h
 

Notes:

1.
s and sh when before a vowel become h

sl becomes l
str becomes tr
sn becomes n (sometimes)

s and sh followed by any other consonant do not change.
 

2.
G may be broad or palatal; in the latter case it is usually written gi, and the lenited form is y,
e.g. giat / yiat, giare / yiare, but the gh spelling is also used, as ghiat, ghiare.

Regular lenition occurs:
 
Variable lenition occurs: