Hi,
Use the
Monbus website and use Fisterra as the destination. Four buses a day out at 9, 10, 13 and 19:00 hours and there are 5 back throughout the day. Last one leaves at 19:00 as well.
To get to Muxia from Finisterre, you need to change bus in Cee but there are not many options so you may find it a squeeze to fit both places in, in the day. Monbus website can again give you the times.
I don't know if there is a bus direct to Muxia from Santiago which may give more favourable timings for a day trip.
The Galician Tourist Office (in the same street and just up from the Pilgrims Office) in Santiago will be able to provide you with further information as I'm sure will others on the forum who may have done this trip in a day.
The other alternative if you are willing to do so, is to hire a couple of cars for the day or even a minibus and split the cost. It may be slightly more expensive than the bus but you can normally get a reasonable day rate and you could be far more flexible with your day. Just a thought!
Cheers
Mig