Nationalism was important during this time in different ways for different countries.
In Italy, nationalism was the major driving force behind unification. People like Garibaldi and Cavour were Italian nationalists and therefore were extremely motivated to unify their country. They wanted to expel foreign rulers and to get all Italian states together in one big country.
Nationalism affected the other countries differently. Nationalism was mainly a negative thing for the Austrians. This was because they held territory in places like Italy where nationalist sentiment was directed against them. Nationalists in those places opposed Austrian rule. Nationalism in other countries affected the French as well. Italian nationalism gave them an opportunity to weaken Austria. On the other hand, Germany nationalism caused the Franco-Prussian War of 1870-1.
Nationalism, then, had different effects on different countries at this time.