Peter O'Brien, 1st Baron O'Brien, PC, QC (29 June 1842 – 7 September 1914), known as Sir Peter O'Brien, Bt, between 1891 and 1900, was an Irish lawyer and judge. He served as Lord Chief Justice of Ireland between 1889 and 1913. In his lifetime he was universally known as Peter the Packer, due to the skill he displayed as Attorney-General in securing verdicts by packed juries.