What is the prefix ir?

Answer:

The prefix means 'not'. So, for example, irresponsible means 'not responsible'.

The prefix, however, is actually in-, such as incapable. You change the n depending on the word. Words that begin with l use the prefix il- (e.g. illegal), words that begin with r use the prefix ir- (e.g. irresponsible) and words that begin with m or p use the prefix im- (e.g. immature or implausible). You use the prefix in- in all other cases.