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 to onion.
Heat a large skillet over medium heat; cook and stir lamb, working in small batches, until browned, about 5 minutes per batch. Transfer browned lamb to onion mixture.
Stir curry into lamb-onion mixture and cook over medium heat for 1 minute. Add water to lamb-onion mixture and bring to a boil. Reduce heat to low, cover saucepan, and simmer until lamb is cooked through and potatoes are tender, about 45 minutes.
Mix coconut milk and chickpeas into lamb-onion mixture and simmer until heated through, about 5 minutes.