irk vs niggle

irk

verb
  • to irritate; annoy; bother 

niggle

verb
  • To fidget, fiddle, be restless. 

  • To use, spend, or do in a petty or trifling manner. 

  • To dwell too much on minor points or on trifling details. 

noun
  • A minor complaint or problem. 

How often have the words irk and niggle occurred in a corpus of books? (source: Google Ngram Viewer )