Classically the liquor is drunk in one gulp and chased immediately by the beer.
How it started? For centuries, distillation was very primitive, leaving all kinds of impurities in the whiskey along with a dreadful taste. So when a man drank a shot, he wash the taste out of his mouth with beer.
The boilermaker originated in the 1890s in the mining camps of Burte, Montana, as the ‘Sean O’Farrell.’
The powerful, ten-cent Sean O’ was served only as miners came off their shift.
Scotland also is a major brewing and beer drinking country. Typically, Scotch ales have been maltier and less bitter than English beers, it became common practice to ‘chase’ a shot of whiskey with light ale.
History of whiskey boilermaker