Check your local colleges and universities; you would be surprised at how many of them have courses in Irish, Scottish Gaelic, Welsh and Breton. Some even offer Cornish and Manx. You could look on the web; there are a lot of language resources there. You can also order books to teach yourself.
In Southern California, you might want to avail yourself of the Irish cultural programs, including language, dance and music, offered by CRAIC.
You might be interested in the Maynooth Celtic Summer School, staffed by leading Celticists and held every third year since 1989 in Maynooth, Ireland. The specifics on this page are out of date, but the contacts work.
Want to learn Modern Welsh? The Cwrs Cymraeg is an 8-day intensive residential language course in Modern Welsh with an emphasis on the spoken language. Tat Cwrs y Mileniwm during July of the year 2000 at Trinity College in Carmarthen. If you have never been to Wales, this is the year to
go. The course is planned for the week before the National
Eisteddfod in Llanelli which is only 15 miles away from the college. Better yet, attendees at the course will be able to stay on at their accomodations at Trinity College for the week of the Eisteddfod as well. Don’t miss this once in a lifetime opportunity to study Welsh in Wales during the first summer of the 21st century.