Porter was fippence a quart following the Napoleonic Wars but prices reduces after that period. I emailed Diageo to enquire about the history of the "price of porter" and I received the following reply:

Dear Mr.Russell, Thank you for your recent email. The Guinness Archive does not hold full pricing details for all the Guinness variants over the years. However, a quart was the equivalent of 2 pints. The earliest pricing information we hold by year is April 1900 3d (old pence) per pint. This would suggest that the song pre-dates 1900. I hope this is of use to you.         Kind regards, Stephanie Driscoll, Consumer Relations.

On the basis of this reply I researched further to discover that Darcy's Porter was sold in Dublin circa 1870 - 1890. The best clue is the word boycott, because this word was coined and entered the Enlish language in 1881, so I presume that the song was written
between 1881 and 1890.

Fergus