Hello Rhiannon,

There is no real specific alignment in my opinion at Calanais I, at least not say with a stone in front and/or some specific horizon feature (like most other monuments: Stonehenge (Heal stone), Newgrange (entrance), Maeshowe (entrance and steep slope), etc.).
At Calanais I it is the skimming of the moon along the horizon (coming out of the Sleeping Beauty, staying low to the horizon and then setting), that is assumed to be important.
This phenomenon can be seen in a large region around Calanais I by the way, so Calanais I is certainly not unique in this!!

Someone else was refering to the reappreaing moon. That also happens, but can't be seen from the main part of Calanais I. But this is visible when is near the southern side (on the small hill). The moon sets behind the Clisham Range to reappear after some time in its valley.
At some other sites this reappearing can be seen from the site, like Calanais II and Calanais III. These sites might be more interesting with regard to that (and less people;-). The biggest sites don't have to be the best sites;-)

By the way the recumbent stone circles in east Scotland (near Aberdeen region) also look to have such a general direction towards this low moon in the south.

All the best,

Victor