meaning - What words sound like opposites but are synonyms?

Though they are of different origin, I would go for 'genius' and 'ingenious', perhaps?

I also have some points to raise from previous answers:

Irregardless is most likely from a combination of 'irrespective' and 'regardless', which are synonyms of each other. Therefore 'irregardless' is seen as an erroneous construction by many, as Sam pointed out.

Also, 'ravel' and 'unravel' are not the same. The phrasal verb 'ravel out' is synonymous with 'unravel' but alone, 'ravel' means to complicate, while 'unravel' in that sense would mean 'make clear something that was complicated'.