Air:
Flights to/from Barra, Benbecula, Inverness, Glasgow, Edinburgh, Aberdeen, London and other destinations operate regularly. British Airways has an online searchable timetable http://www.flybe.com/
Vehicle ferry:
During the summer, a ferry sails from Ullapool on the mainland to Stornoway 2 or 3 times a day including Sundays. The service is slightly reduced during the winter. Journey time: 2 hours 40 minutes. Vehicle reservation required.
The official Caledonian Macbrayne website http://www.calmac.co.uk lists the current timetables in detail.