Start by navigating Safari to the app: http://whereweare.org/tiree/twc/m
Then press the middle button at the bottom of the screen.
A menu appears. You are probably familiar with it; it is the same menu you use to bookmark or share a web page.
Select the option "Add to Home Screen".
You are then asked for a name for the application. The default is "Tiree Map", but you can choose anything you like.
The Tiree Map app will then appear as an icon on your iPhone/iPad home screen :-)
This will usually be at the end of the last page if you have many apps.
The name will be whatever you gave it at step 3.
Now, simply select this whenever you want to use the app.