Your friend is right, starting in Irun means you will be walking beautiful stages - and probably end up somewhere around Santander. However, it takes a full day of travelling to get to Irun (which means one day less for walking), and to bus/train back from Santander also takes 8 hrs.
If you want to starting walking 300 km from Santiago, take a bus to Gyon (takes 5 hrs) and walk to SdC.
Two other options (entailing app. 300 km of walking) that you may want to consider:
- Bus or train to Leon, and walk the
Camino Frances to SdC.
- Bus or train to Porto, and walk the Portuguese to SdC (a bit less than 300 km, but of course you can walk on to Muxia).
There is pros and cons to all options. So you may end up tossing a coin
Happy planning !