CHRISTOPHER SOMERVILLE is one of Britain's best-known travel writers, with some 25 books to his name including the best-selling National Geographic Traveler: Great Britain. His love affair with Ireland was kindled on his first trip, an 800-mile walk through the rugged, romantic west. He has since written four books about the island, and he returns there whenever he can to see old friends, walk the mountains and coasts, and play traditional music.