The most common and most likely answer to this crossword clue is the 8 letter word RATTIEST
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More clues leading to the result RATTIEST
Buddy explains!

The straight part
We know that "Most irritable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most irritable = rattiest.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RATTIEST mean?
RATTIEST adjective- Similar to a rat; ratlike.
- Infested with rats.
- In poor condition or repair
- Irritable, annoyed, bad-tempered.