Both Scottish and Irish are Celtic. Celtic refers to both language and culture derived from the Celts, a group of tribes that existed in Europe during the Iron Age through Medieval times.
Ireland, Scotland, Brittney, Wales, Cornwall and the Isle of Man are collectively known as the Celtic Nations. Each of these areas has a language that can be traced back to the Celtic tribes. Areas of Canada, including Newfoundland, Nova Scotia and Cape Breton, and portions of Argentina also have languages with Celtic roots. Portions of Spain and Portugal have aspects of Celtic culture, but they do not have a language that can be traced to the tribes.