When switching from a angular project to an Ionic project you have to know the following, because otherwise your Ionic icons might not work. To the "projects" -> "architect" -> "build" -> "assets" of your angular.json file, add: 

              {
                "glob": "**/*.svg",
                "input": "node_modules/ionicons/dist/ionicons/svg",
                "output": "./svg"
              },

From then on, you can just use the name of the icon to show it:

<ion-icon color="dark" name="cloud-offline-outline"></ion-icon>