irregular vs occasional

irregular is a noun but occasional is not a noun.

Word	Noun	Adjective	Verb	Adverb
irregular	Yes	Yes	No	No
occasional	No	Yes	No	No

irregular (noun)	occasional (noun)
merchandise that has imperfections; usually sold at a reduced price without the brand name
a member of an irregular armed force that fights a stronger force by sabotage and harassment

irregular (adjective)	occasional (adjective)
contrary to rule or accepted order or general practice	occurring from time to time
(of solids) not having clear dimensions that can be measured; volume must be determined with the principle of liquid displacement	occurring on a temporary or irregular basis
(used of the military) not belonging to or engaged in by regular army forces	recurring or reappearing from time to time
deviating from normal expectations; somewhat odd, strange, or abnormal	occurring or appearing at usually irregular intervals
failing to meet a standard of manufacture due to an imperfection
lacking continuity or regularity
not occurring at a regular rate or fixed intervals
independent in behavior or thought
(of a surface or shape); not level or flat or symmetrical