When Barnard visited in the 1880's, there were 8 breweries in the town, which had a population of just 11,000. Like many other Scottish businesses, the brewers of Alloa rode on the coat-tails of the British Empire, exporting beer to distant colonies.
"As far back as the year 1745, we find by the Kirk Session Records that George Younger was a brewer; and his name and business have been handed down in unbroken line to his descendants, the present partners. It is more than a century since Alloa ale was first exported to the Indies and the Colonies, the trade having commenced at the time when the town was the most important seaport on the Forth. In those days it was customary for the captains of the vessels to take out with them on their voyage, fifteen or twenty casks of ale, bringing the money back for it when they returned home.
It is on record that in the year 1865, Mr. James Younger, the father of the present partners, who had considerably increased the trade of the brewery under his active management, did, in that year, the largest export trade, and the most extensive bottling business of any brewer in Alloa. But it is due to the present enterprising partners that the business has attained, under their spirited management, the position of the largest brewery concern in Scotland outside Edinburgh.
The change in the popular taste from heavy strong ale to the light and more sparkling beverage, was quickly taken advantage of by Messrs. G. Younger & Son. Anticipating the demand, in an incredibly short time, they had succeeded in placing their East India pale and bitter ales in the foreign markets, until they were in such demand as to necessitate an extension of premises."
"Noted Breweries of Great Britain and Ireland, vol. 2", Alfred Barnard, 1889, pages 429 - 431.
I've said this before. And I'll keep on saying it until people listen: lots of IPA was brewed in Scotland. I know it doesn't fit well with the usual fantasy of Scottish beer being a smoky, barely-hopped brew. But there you go. Just because it's inconvenient for some theories doesn't make it any less true.
And, of course, Scottish brewing wasn't static. 19th century Britain was a dynamic place. You had to keep up with current trends to stay in business. Just as London's Porter brewers moved into Ale and Pale Ale production in the middle of the century, the more adventurous Scottish brewers got into Pale Ale in a big way.
"the largest brewery concern in Scotland outside Edinburgh" isn't quite as impressive as it might sound. As I've demonstrated before, 50% of Scottish beer was brewed in Edinburgh. Alloa was the only other significant centre of brewing. In some large cities - Glasgow is a good example - there was surprisingly little brewing.