As I am in Santiago twice each year, I compiled this list of all the autoservicio lavanderias (coin laundromats) I have come across, and which are convenient to the Old Town:
Lavanderias- Coin Operated - Santiago de Compostela
CE Colada Express
Santiago de Quayaquil #12
Newtown, left off Rua do Horreos
7-days: 09:00 - 22:00
H10
Rua das Hortas #10
Bottom of ramp, across from St. Fructoso. Closest to P/O.
7-days: 08:30 - 22:00
Wi-Fi
Waterproof Lavanderia Autoservicio
Rua da Preguntoiro #17
Off Praza da Cervantes
Opposite Last Stamp albergue
7-days: 07:30 - 23:00
Wi-Fi
SC18
Rua da San Clemente #18
Behind National Police station
Right side, going up curved street / hill.
7-days: 08:30 - 22:00
Wi-Fi
A Casa do Peregrino
Rua Azabacheria #2
Before Praza da Cervantes, in left. Ice cream counter out front. Two washers, 2 dryers, in basement. Buy tokens at Hostal counter inside.
The list is current through last week...
BTW, in addition to this, Pilgrim House, at Rua Nova #19 offers laundry service, wash, dry and folded for €6 per load. This is a bargain compared to the autoservicio places above. But, it does take longer.
Hope this helps.