Every time you read a story in Dubliners, pretend for a second that the title of that story is "Dubliner." You'll get a sense of each story as a case study of one (or sometimes two or three) person who inhabits the city. But you'll also get a sense of how each story really focuses on how strange and peculiar each situation really is. These Dubliners spend a lot of time in their own minds, trying to figure things out for themselves, even when they're in a room full of people. So many stories in Dubliners feature epiphanies—private revelations and discoveries—and these important realizations happen in their own minds, rather than through contact with other people. But does this mean that the characters are lonely and isolated, or just really really private?
The difference between good old-fashioned solitude and the Dubliners brand of isolation is that the second drives characters to anguish, poor decisions, and harm to themselves and others.
Strangely (or totally obviously), love is one of the great causes of isolation in Dubliners. It's true for the narrator of "Araby," for Mr Duffy, and for Gabriel Conroy.