This is going to be a boiled-down response, if that's okay, since it's such a huge topic.

First, I'm assuming you're referring to British nationalism/industrialism as you're right to point out that these were more or less coterminous phenomena in Britain. So, nationalism was already on the rise pre-Industrial Revolution for several reasons: British expansionism and voyages across the globe resulted in domestic notions of what it meant to be a 'civilized people.' Aided by the natural philosophies of taxonomies (i.e. the Linnaean system of ordering the natural world) and travel reportage (writing back to the 'nation' or the Royal Academy of Sciences), ideas of what were 'other' and what were 'home' helped lead to a sense of nationalism that in some ways appeared organic and homegrown. Mary Louise Pratt (if you're interested) calls this "planetary consciousness" in her book Imperial Eyes: Travel Writing and Transculturation. Essentially she argues that this vision of the world with all its discrete parts allowed for clearer distinctions between peoples and nations. Mixed with Britain's imperial ethic, this helped nationalism grow to what it came to be by the time of the Industrial Revolution.

Fast forward to the IR: we see a massive shift from rural cottage industries to factories, for which modern cities were designed to house this huge influx of people. With industrialization (and here I'm going to simplify), there was a growth in the types of socialization that were not/rarely seen prior to the movement to cities. Literacy rates boomed resulting from the printing press and the relative affordability of newspapers; news from other towns could be gleaned also with relative ease (the rise of the Royal Mail and the affordability of stamps), and even from far away due to the rapid expansion of train lines. Coffee houses became a thing--people could gather here, even (and maybe especially) those couldn't yet read, and everyone could discuss and argue about the news of the day, particularly in response to domestic politics but also international relations. This ability to relate information led to a more specific--and recognizable--growth in national identity: people could tease apart the finer points about what made them a 'British people,' as they drank their tea (an import of Eastern colonization--another marker of the British national identity at that time) and laboured under their urban employment.

Obviously, the zones of interaction between these two events--nationalism and industrialization--are quite complex, but I hope this has given you some idea of how the two are connected.