Because Jen Black made a comment about Irish whiskey v Scotch whisky, I did some basic research. Please note, the American Regencies that have bothered me ALL have made it clear that it was Scotch whisky the Regency rakes were drinking. IF they had said, Irish, I would have believed them and would have applauded their research.
The two biggest Irish whiskey distillers were licensed in the 1780's. This is the period that Highland distillaries were prohibitted from exporting and possibly contributed to their growth. John Jameson and sons in Dublin and Bushmill's in the North. John Jameson's motto -- Sine Metu -- Without fear is still printed on every bottle of Jamesons. In the Book of Lienster, there is a record of whiskey being drunk in the 12th century. The first license for Bushmill's was given in 1620 by James I.
Jameson's was the second biggest distiller in Ireland by 1820. However, both websites admit the real rise in the consumption of whiskey did not happen until 1858-1893 when the Phylloxera hit the grape vines of Europe, nearly wiping out wine and brandy production. In 1890, Irish whiskey accounted for 90% of the worldwide whisk(e)y market with Jameson providing 10% of that. They were severely hit by US prohibition.
Irish whiskey tastes slightly different to Scotch single malt. I am not sure when it got its colour, but suspect that it had to do with port or sherry casks.
Thus, if Regency rakes do drink whisk(e)y, it is probably best if it is Irish, and possibly served in a punch or a toddy. But really, it was not as popular fortified wine, or brandy or even rum or gin.