Intrinsic (adj) part of the essential nature of something; inherent

His intrinsic boldness was always getting him into trouble.

Introspective (adj): tending to think about oneself; examining one’s feelings     

Introspective examination of his motives led him to conclude that he must have been at fault in the breakup of his marriage

Inundate (v) to flood, to cover completely with water, to overwhelm

Tiny island kingdom was inundated by the tidal wave.

Invective (n): insulting or abusive speech

He wasn’t much of an orator, but he was brilliant at invective.

 

The reviewer was filled with bitterness and invective.

Inveterate (adj) habitual; firm in habit; deeply rooted

His habit has inveterate.  

Irascible (adj): easily angered or provoked; irritable

A grouch is irascible.

Ironic (adj): meaning the opposite of what you seem to say; using words to mean something other than what they seem to mean.

He seems to be ironic.

Irrevocable (adj): irreversible, irreversible, immutable, unalterable,

Something irrevocable cannot be taken back

Revocable: something that can be reversed

Itinerant (adj): moving from place to place

The life of traveling salesman is an itinerant one.

Itinerary: planned trip route.