The narrator wonders aloud as to the contents of Coronation Ale: "What was in this Coronation Ale, offered in a dark brown bottle of a shape narrower and more elongated than the beer bottle of later days, with 'Atkinson--Gildsey' embossed upon it and a label bearing a large crown, centre, and a continuous border of alternating smaller crowns and union jacks? Nectar? Poison? Merriment? Madness? The bottled-up manias of His Majesty's subjects?" (p.129) Whatever the particular ingredients, a single bottle of this brew takes one "with astonishing rapidity through the normally gradual and containable stages of intoxication: pleasure, satisfaction, well-being, elation, light-headedness, hot-headedness, befuddlement, distraction, delirium, irascibility, pugnaciousness, imbalance, incapacity -- all in the gamut of a single bottle." (p.129)
The description of Coronation Ale's effects is interesting outside the context of the Coronation because it in many ways (delirium, imbalance, incapacity) reflects the progress of mental illness, especially as portrayed by Swift. This comparison yields a direct link between drunkenness and insanity, most importantly in the area of "relative" insanity. The question of who is sane (and who insane) is at issue in almost all of Swift's writing. The same questions can be asked in relation to Coronation Ale. Who does it affect, and is it really the potent intoxicant it appears? The answer is found in an old aphorism, "it takes more than his own beer to get a brewer drunk." (p.130) Taken metaphorically, this is to say that what may appear crazy to one person is perfectly normal to another. Ernest's Coronation Ale causes madness in all who drink it -- everybody except himself that is.
A side issue that you may want to consider: is Ernest Atkinson crazy? If so, can you point to a time in the novel when he crossed the boundary from sanity to insanity? How would you have to change your interpretation of the above quotation about brewers and their own beer if you decide that Ernest is insane?