We have compared the plasmonic enhancement of second-order nonlinear optical (NLO) properties in two types of ionic self-assembled multilayer (ISAM) films combined with Ag nanoparticles fabricated using nanosphere lithography (NSL). The light-concentrating properties of the Ag particles lead to a marked increase in the NLO efficiencies of thin ISAM films. The induced enhancement is found to be much larger in conventional ISAM films than in films made with the hybrid covalent ISAM technique (HCISAM), even though the latter have a significantly larger intrinsic bulk second-order nonlinear susceptibility $\left({\chi }^{\left(2\right)}\right)$ . The plasmonic enhancement of NLO effects is shown to be primarily an interface effect due to the short decay length of the plasmon modes. The importance of interface effects in the films has been investigated by surrounding thin ISAM and HCISAM films with NLO-inactive buffer layers, which confirmed the important role played by the interfaces, particularly for the HCISAM films.