deadening vs irksome

deadening is a noun but irksome is not a noun.

Word	Noun	Adjective	Verb	Adverb
deadening	Yes	Yes	No	No
irksome	No	Yes	No	No

As adjectives, deadening and irksome are synonyms defined as:

deadening and irksome: so lacking in interest as to cause mental weariness

Other synonyms of deadening include boring, dull, ho-hum, slow, tedious, tiresome, wearisome.

deadening (noun)	irksome (noun)
the act of making something futile and useless (as by routine)

deadening (adjective)	irksome (adjective)
so lacking in interest as to cause mental weariness	so lacking in interest as to cause mental weariness