Specifically what led to Irish Red Ale becoming a style all its own? Did it develop from the type of ingredients used or was it based out of necessity from the import of highly taxed English ingredients? Both the Irish and Scottish developed similar beer styles as a result of English laws regulating which ingredients could and could not be used in the making of beer and high taxes forced them to be used sparingly. Could the lack of ingredients from high taxes have been the source for Irish Red Ale becoming a style?
The taxes you're referring to I'm assuming are the ones relating to malted barley. It is absolutely the case that Irish brewers specifically used unmalted roasted barley in recipes to reduce costs. Interestingly, in England you paid taxes either way. Roger Protz has written about this and how Guinness is famous for skirting those taxes and undercutting competing stouts in London pubs.
Red has its roots in Pale Ale. In the 19th century, Irish brewers were in rough shape due to famine and the temperance movement. Irish drinkers liked Pale Ale, and they were drinking good stuff from England. Irish porters and stouts were mostly being consumed abroad.
So it makes sense that locals started making their own Pale Ales with Irish strains of ale yeast and variations in ingredients (unmalted roasted barley) and they just kinda evolved until people latched onto it and identified it as a style.
Strangman, Davis and Co. of Waterford, represent a vast and influential trade in Ireland, by the exhibition of two casks of Ale and Porter. It would be rather a difficult matter to express any opinion as to the character of the liquor from the appearance of the vessel in which it is contained; besides, I must admit my total incompetency to pronounce a judgment, in any shape, upon an article which is so popular, and so largely consumed. It is only necessary to say, with respect to the Irish brewing trade, that it is recovering from the serious injury which it sustained by the temperance movement, and the renewed depression in the years of famine; that its home consumption is very little interfered with by English importation—Bass' pale or bitter Ale being the chief article which finds anything like favour in this country; and that the Irish brewers, on the contrary, do a large and increasing business in England, and with foreign countries. Cork Porter, Drogheda Ale, and Dublin Stout, are well known and highly appreciated out of Ireland. The firms of Beamish and Crawford, and Lane, of Cork, and Guinness of Dublin, are amongst the most eminent in the United Kingdom. Abbott of Cork also enjoys a high reputation. There are, besides, several prosperous breweries in the principal towns throughout the country. Kinsale, Bandon, Fermoy, Youghal, Dungarvan, and many others in the neighbouring counties, have each their thriving brewery.
John Francis Maguire (1853) The industrial movement in Ireland, as illustrated by the National Exhibition of 1852.