It is possible to get to Callanish as a foot passenger from Ullapool... I did that in 2002, getting the bus from Stornoway for about 4 hours on site before catching the last ferry back. There's a YH at Ullapool.
Needless to say I had to come back in 2004 touring The Uists and Lewis by car. Note that there are some wonderful YHs there too... thatched black houses. Recall waking up in one (Tobha Mor) to find a bird sutting on the bunk looking at me.
