Slow hydrolysis is often limited by mass transfer and inefficient electron transfer in the anaerobic digestion (AD) of straw. In this study, a microbial electrolysis cell coupled with anaerobic digestion (MEC-AD) system was constructed to enhance methane production using Fe-doped birnessite (Fe-Bir). Fe-Bir materials were synthesized with varying manganese-to-iron molar ratios (1:0.01, 1:0.05, and 1:0.1) via ion co-precipitation. Transition metal ions (Fe³⁺, Mn²⁺) were mixed with an alkaline KMnO₄ solution. The material with a Mn:Fe molar ratio of 1:0.05 demonstrated optimal overall properties after characterization. This optimal Fe-Bir was subsequently added to the corn straw MEC-AD system at different dosages (1%, 3%, and 5%). A systematic investigation was conducted to explore the synergistic and underlying mechanisms of Fe-Bir with MEC for methane production. The results showed that the superior performance of the Fe-Bir synthesized at a Mn:Fe ratio of 1:0.05. The material exhibited an increased cyclic voltammetry (CV) curve integral area, a specific capacitance of 124.41 F/g, a charge transfer resistance of 27.95 Ω, along with an enhanced specific surface area, pore size, and pore volume, as well as excellent thermal stability. In the bioreactor trials, the system amended with 3% of the optimal Fe-Bir (designated as the MEC-FeB3% group) achieved the highest methane yield of 255.84 mL/g using volatile solids (VS). There were significant increases of 65.58% and 37.73%, compared with the conventional anaerobic digestion control (AD group) and the standalone MEC-AD system (MEC group), respectively. The chemical oxygen demand (COD) removal rate in the MEC-FeB3% group reached 71.64%. Microbial community analysis revealed that specific taxa were substantially enriched in the integrated systems. Notably, the acetoclastic methanogen Methanothrix and the bacterial genus DMER64 were enriched significantly. Compared with the MEC and AD control groups, the relative abundance of Methanothrix increased by 108.98% and 37.27%, respectively, in the MEC-FeB3% group. The enrichment of Methanothrix, an obligate acetoclastic methanogen, indicated a major reinforcement of the acetoclastic methanogenic pathway concurrent with a potentiation of direct interspecies electron transfer (DIET) within the bioaugmented system. Metagenomic functional analysis showed an upregulation of key enzymes involved in methanogenesis. Crucially, the gene encoding acetyl-CoA synthetase (EC6.2.1.1) was significantly upregulated in the methane metabolism pathway, thus promoting the conversion of acetate to acetyl-CoA and thereby enhancing both acetoclastic methanogenesis and the DIET process. Consequently, the dominant methanogenic pathway in the enhanced system evolved from a primarily hydrogenotrophic mode to a highly efficient, synergistic model co-dominated by hydrogenotrophic methanogenesis, acetoclastic methanogenesis, and DIET. In summary, Fe doping effectively modified the birnessite structure to modulate the interlayer ion arrangement, thus increasing the surface oxygen vacancy concentration for the average manganese oxidation state. These modifications collectively enhanced the electron transfer and interfacial reactivity. Within the MEC-AD consortium, the optimal Fe-Bir functioned dually as an efficient extracellular electron shuttle and a DIET facilitator. It promoted the hydrolysis and acidogenesis stages to accelerate the conversion of metabolic intermediates, ultimately establishing a robust, multi-pathway methanogenic network where hydrogenotrophic, acetoclastic, and DIET-based routes operated in parallel and synergistically. Multi-channel functional architecture improved the system's resilience to fluctuations in electron donor availability and substrate composition, thereby significantly enhancing the overall stability, efficiency, and methane yield of the anaerobic digestion. These findings can provide the technical strategy and theoretical reference for the efficient anaerobic digestion of straw.