Place lamb in a bowl and drizzle 2 teaspoons olive oil over lamb; stir until coated.
Heat remaining 2 teaspoons olive oil in a saucepan over low heat; cook and stir onion until softened, 10 to 15 minutes. Remove saucepan from heat and add potato cubes.
Heat a large pan over medium heat; cook and stir lamb, working in small batches, until browned, about 5 minutes per batch. Transfer browned lamb to onion-potato mixture.
Stir in curry powder and cook over medium heat for 1 minute. Add water and bring to the boil. Reduce heat to low, cover saucepan, and simmer until lamb is cooked through and potatoes are tender, about 45 minutes.
Stir in coconut milk and chickpeas; simmer until heated through, about 5 minutes.