I believe the API is required for flying in Spanish airspace, therefore I presume its irrelevant whether you are leaving or arriving in spain. Globespan will probably take the information at check-in (this has happened for me with several other airlines), however Ryanair ask for it on-line. I guess this is to help keep costs down as you submit it yourself.
Not sure if it applies to all Ryanair flights, but the last time I used them to fly to spain, anyone who had not provided it on-line had to provide it to the service desk, which meant queueing twice, as they didn't take it at the check-in desks. Some people didn't bother, and as a result, a list of at least 20 names were read out on the plane, and we couldn't take off until all of them had provided the info to the cabin staff. It took that long, we missed our take off slot, although thankfully another was available pretty quickly.