Dalriada was originally the name of a kingdom in Northern Ireland which - in the 5th century - set up a colony in Argyllshire in Western Scotland. By the 9th century, Viking invasion had weakened the Irish side, so that the kingdom thereafter existed only in Scotland, where it had taken more or less complete control of the territory. The original Irish people were called 'Scots' - yes, I know it sounds odd! - but that's where the name Scotland actually came from.
My apologies for omitting the answer to the second part of your question in my earlier response. 'Dalriada' probably means 'the race of long-armed ones'.