In this paper, we propose IrOnTex, an approach to design, fabricate, and prototype customizable interactive textiles by ironing 3D printed elements directly onto fabrics. Our method incorporates multiple filament types to demonstrate functionalities such as electrical conductivity, sensing, and thermo- and photo-sensitivity for interactive applications. Additionally, we integrate a tiling-like fragmented design to improve stretchability and flexibility to match the mechanical behavior of textiles in our 3D printed elements. We characterized the iron-on process through technical evaluations, which showed the durability and reversibility of the approach. We also suggested baseline parameters for prototyping and methods to enhance the functionality of certain materials. The empirical results from the studies were used to develop a design tool to assist the design process. A user study with 10 participants revealed the helpfulness of the tool. To demonstrate the capabilities of the approach, we show a set of example interactive applications. We envision the proposed approach will enable designers to rapidly prototype interactive textile applications.