Traill was born in Lisburn, County Antrim. He earned the degree of Doctor of Divinity and afterward, in 1832, was appointed the rector of Schull, County Cork. He antagonised some local people with his fervent evangelical Christianity. He translated some of the manuscripts of Josephus, a first-century Jewish Historian, into English.
Traill is said to have discovered copper at the Dhurode mine on Mizen Head which first operated between 1844 and 1846. He was a major shareholder in the mine and one of its six shafts was named after him.
At the outbreak of the Great Famine in 1846 Traill believed that the storage of potatoes in pits would save them from the blight and worked on constructing these from October 1846. However he realised this would not be successful and by December was trying, in vain, to persuade the local landlords to let their tenants keep some grain so that they weren't forced to eat their seed potatoes. Traill established a relief committee for his parish and wrote widely to persuade people to subscribe to it. He was shown in the Illustrated London News visiting a dying man and his family, having been sketched by James Mahoney who said of Traill that "his humanity at the present moment is beyond praise".
Traill died of "famine fever" (typhus) in 1847. He left a large family including two sons, three-year-old Robert Walter and baby Edmund. The family moved to Dublin, where Robert studied civil engineering and Edmund Medicine at Trinity College before they abandoned their studies to become ranchers in Argentina. Robert Walter Traill's son was Johnny Traill, the noted polo player. Another of Traill's grandsons was John Millington Synge, the Playwright. His great-great-great granddaughter is TV Producer and Writer Daisy Goodwin. Goodwin wrote Traill into an episode of ITV's Victoria which told the story of the Great Famine. Traill was played by Martin Compston.
