Train travel within Spain and Portugal is quite good, reasonable and useful. However, train links between the two countries are, at present, not good.
Bus Travel, however, is probably the best way to travel between the two countries. It is unlikely that you can get from San Sebastian, Spain to Lagos, Portugal in one day. It can be done driving - a very long day! According to Google Maps the distance is 1,181 km or about 11 hours 13 mins drive.
There are no direct bus services between San Sebastian and Lagos. You will probably need to route via Lisbon.
www.alsa.es have two daily departures from San Sebastian to Lisbon Sete Rios bus station changing in Madrid:
Dept. Arr:
S.SEBASTIAN 00:30 LISBON S.R 17:00 Price: 74,54
S.SEBASTIAN 09:00 LISBON S.R 23:00 Price: 74,54
From Lisbon Sete Rios bus station there are regular bus services to Lagos throughout the day. Price one way around € 17:00 each.
You can find an extract of the timetable here on my blog:
http://barlaventostravelblog.blogspot.com/2009/02/lagos-to-lisbon-buses-rede-expressoseva.html
