Tirade in Irish islands ultimately scathing in tone (8)
I believe the answer is:
harangue
'tirade' is the definition.
(I've seen this before)
'irish islands ultimately scathing in tone' is the wordplay.
'irish islands' becomes 'aran' (I've seen this before).
'ultimately' suggests the final letters.
'in' indicates putting letters inside.
'tone' becomes 'hue'.
The last letter of 'scathing' is 'g'.
'aran'+'g'='arang'
'arang' put within 'hue' is 'HARANGUE'.
'in' is the link.
(Other definitions for harangue that I've seen before include "A lengthy vehement speech, may be abusive" , "Pompous speech" , "Forceful and aggressive speech" , "A lengthy speech or diatribe" , "address" .)