New brominated β-carbolines irenecarbolines A (1) and B (4) along with known β-carbolines 2 and 3 and a new 8-oxoisoguanine derivative, 5, were isolated from a solitary ascidian, Cnemidocarpa irene. The structures of these compounds were determined on the basis of their spectral data. All, except for 3, inhibited the action of acetylcholinesterase (AchE). The activities of 1 and 5 were comparable to those of galantamine, a clinically used AchE inhibitor. Compounds 1 and 2 were found to be present in high concentrations in blood, and fluorescence was observed in certain types of cells found in the blood of the tunicate.