Heat the olive oil in a large Dutch oven over medium-high heat. Add the lamb cubes and brown them on all sides, about 6 minutes total. Remove the lamb and set it aside.
Add the onion to the same pot and cook until softened, about 4 minutes. Stir in the garlic, cumin, coriander, cinnamon, paprika, and cayenne. Cook for 1 minute until fragrant.
Pour in the Guinness and scrape up any browned bits from the bottom. Return the lamb to the pot along with the diced tomatoes, apricots, chicken broth, salt, and pepper. Bring to a simmer.
Cover and cook on low heat for 1 hour and 30 minutes, stirring occasionally, until the lamb is fork-tender.
While the stew simmers, prepare the couscous according to package directions using 1.5 cups boiling water or additional broth. Fluff with a fork when done.
Ladle the stew over portions of couscous and top with chopped cilantro before serving.