Tir nan-Og strangely left out of the picture? (8)
I believe the answer is:
ignorant
'left out of the picture?' is the definition.
(being out of the picture means being ignorant)
'tir nan-og strangely' is the wordplay.
'strangely' indicates anagramming the letters.
'tir'+'nan'+'og'='tirnanog'
'tirnanog' with letters rearranged gives 'IGNORANT'.
(Other definitions for ignorant that I've seen before include "Lacking knowledge or skill" , "Uncouth or uninformed" , "No rating for being uninformed" , "Unaware, uninformed" , "Poorly-educated" .)