The occurrence of irregularly variable RR Lyrae stars in four globular clusters and the Draco dwarf spheroidal galaxy are examined in order to determine their period distributions. Two of the clusters are of Oosterhoff type I, M3 and M5, and two are of Oosterhoff type II, M15 and omega-Cen. Draco does not fit into either Oosterhoff group, having a medium ab-type period of 0.611 days, but few C-type variables. Irregular variability is more frequent among the shorter period ab-type RR Lyraes (P less than 0.65 days) and, at least in M15 and omega-Cen, among the longer period c-type variables. These period distributions are consistent with some mode-mixing explanations for irregular variability. It is suggested that the Blazhko effect in ab-type RR Lyraes is a nonlinear mixture of the fundamental and third overtone radial pulsation modes (Borkowski, 1980). This accounts for the occurrence of irregular ab-variables only at shorter periods, but yields RR Lyrae star masses larger than those predicted by stellar evolution theory.