Rapid prototyping of interactive textiles is still challenging, since manual skills, several processing steps, and expert knowledge are involved. We demonstrate Rapid Iron-On User Interfaces, a novel fabrication approach for empowering designers and makers to enhance fabrics with interactive functionalities. It builds on heat-activated adhesive materials consisting of smart textiles and printed electronics, which can be flexibly ironed onto the fabric to create custom interface functionality. To support rapid fabrication in a sketching-like fashion, we developed a handheld dispenser tool for directly applying continuous functional tapes of desired length as well as discrete patches. We demonstrate versatile compositions techniques that allow to create complex circuits, utilize commodity textile accessories and sketch custom-shaped I/O modules. We further provide a comprehensive library of components for input, output, wiring and computing. Three example applications demonstrate the functionality, versatility and potential of this approach.