We study the combined impact of magnetic mirroring and focusing on the ionization by cosmic rays (CRs) in dense molecular clouds and circumstellar disks. We show that for effective column densities of up to ∼1025 cm-2 (where ionization is the main mechanism of energy loss by CRs) the two effects practically cancel each other out, provided the magnetic field strength has a single peak along field lines. In this case the ionization rate at a given location is controlled solely by attenuation of interstellar CRs due to energy losses. The situation is very different in the presence of magnetic pockets—local minima of the field strength, where the CR density and thus ionization can be reduced drastically. We obtain simple analytical expressions allowing accurate calculation of the ionization rate in these regions.