The most obvious is 1. ...Qxh2+ 2. Kxh2 Ng3+ 3. Kg1 Rh1#
But I'm pretty sure this is not a forced mate, because the king doesn't have to take the queen sac. 1. ...Qh2+ 2. Kf1 Ng3+ 3. Ke1 and the king escapes. Black is winning fairly easily, but it's no forced mate.
Should be forced either way, perhaps you didn't analyse deeply enough?
There's really no forced mate there after 2. Kf1. Maybe you are overlooking something in your own analysis? The queen check on h1 leads to the same sequence of running away. The light-square bishop can't check on g4 because of the knight, and even if it could, there's plenty of piece sacrifices to save the king in the short term. If there's a forced mate, it's something like mate in 20, which is silly. White has to cooperate with the queen sacrifice to get mated by black.