irksome vs deadening

irksome is not a noun while deadening is a noun.

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

As adjectives, irksome and deadening are synonyms defined as:

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

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

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

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