His education was interrupted by the American Civil War, when Irwin served with the 45th Iowa Volunteer Infantry Regiment, and reached the rank of sergeant.

The couple had three children, Alice, Elizabeth, and John R.[2] Despite his legal training, Irwin spent most of his efforts working at the family dry goods business.

[4] Following the removal of John Baldwin Neil, Irwin was nominated as Governor of Idaho Territory by President Chester A. Arthur and commissioned on March 5, 1883.

By October, the new governor was expressing his desire to soon return to his duties while at the same time expecting to undergo surgery to correct the problem with his eyes.

[6] An unusual aspect of his tenure was Irwin returning most of his salary, claiming he could not in good conscience accept the unearned pay.

[7] This action received widespread newspaper results due to its rarity – most political appointees of the day took all monies they could collect without question.

[4] Following the resignation of Lewis Wolfley as Governor of the Arizona Territory, President Benjamin Harrison took over a month to choose a replacement.

[8] The new governor was however delayed in assuming his new position, being quarantined in his home after his son was diagnosed with scarlet fever till January of the next year.

Secretary Oakes Murphy, as Acting Governor, had already given the session's opening address, so Irwin made his wishes felt through use of his veto.

To help control the territorial deficit the governor vetoed an "outrageous" appropriation bill, forcing the legislature to pass a less ambitious spending plan.