Kevin;
I'm married to a Murphy so we've done some research on the family. If your Murphys are from the south of Ireland, then MacArthur would be acceptable as the Murphys in County Cork were a sept of the Irish MacArthur Clan. If you family came from the east of Ireland they were a sept of the Scottish McDonald Clan and the MacArthur tartan wouldn't be appropriate.
Hope this helps.
BD