In the song "Whiskey in the Jar", would Captain Farrell have been British or Irish? What was the historical setting and context of the song's tale?
"Whiskey in the Jar" (see Wikipedia and LiveAbout) is a traditional Irish folk song about a man who robs an army officer and then is betrayed by his lover. Originally an oral tradition with many variations, it has been recorded by various bands and singers in the 20th century.
The origin of this song would have been during the time of British rule in Ireland, whether the Lordship or Ireland (pre-1542, dominated by feudal English lords with some regions still controlled by Gaelic chiefs), the Kingdom of Ireland (1542-1800, ruled by the English or British monarch, de jure as a separate state but largely dominated by English nobles) or the United Kingdom of Great Britain and Ireland (post-1800, part of a unified state). According to Alan Lomax's The Folk Songs of North America, in the English Language, this song likely inspired the 1728 work The Beggar's Opera, which would place its origin in the 18th century or earlier. Given the use of captain and the apparent organisational structure of the armed forces, as well as the use of pistols, it must also come from the 16th century or later. That sets the origin of the song during the Kingdom of Ireland. During this time there was an Irish Royal Army, which mostly consisted, at least in its leadership, of Englishmen stationed in Ireland, although there were always many Irishmen serving as soldiers.
The song "Whiskey in the Jar" has a rebellious theme, with the Irish rogue robbing the rich officer. For this reason it became popular in colonial America, specifically for the theme of rebellion against the British. This is evidence for Captain Farrell being a British officer. On the other hand, the name Farrell is an Irish one, originating from the clan of Ó Fearghail. This is evidence for Captain Farrell being originally Irish.
However, whether he came from English or Irish roots himself, by serving as an officer in the British Army Farrell represents the oppression of Ireland by the English, either as a colonist or as a collaborator. This strengthens the theme of rebellion in the song: it's not only a highwayman robbing a rich man, but an Irishman robbing a British Army officer. Even if Farrell was also Irish, he may have been "Brittified", representing the rule of Britain and not the spirit of Ireland. The context of this song is set against a long history of Irish rebellions against the British. Even if he wasn't part of an uprising, the highwayman in "Whiskey in the Jar" represents the same spirit of Irish rebellion.
Of course it's also possible to interpret the song at face value: a simple story of a man who robbed an army officer and was betrayed by his lover. Perhaps that's what it was when first composed, and how many singers and listeners thought of it. But it would certainly fit in with the long tradition of Irish folk songs that are more explicitly about rebellion against the British, more recent ones including "Come out ye Black and Tans" and "Kinky Boots" (both gloriously cheeky). Although less openly rebellious and anti-British, "Whiskey in the Jar" can be, and has been, interpreted as part of the same tradition.