Lacking knowledge of Tir-nan-Og at sea (8)
I believe the answer is:
ignorant
'lacking knowledge' is the definition.
(I've seen this before)
'tir-nan-og at sea' is the wordplay.
'at sea' is an anagram indicator.
'tir'+'nan'+'og'='tirnanog'
'tirnanog' with letters rearranged gives 'IGNORANT'.
'of' acts as a link.
(Other definitions for ignorant that I've seen before include "Unlearned, lacking knowledge" , "Poorly-educated" , "Discourteous" , "Lacking in or good manners" , "No rating for being uninformed" .)