Algal biomass is attracting more and more interest due to new potentials for overcoming the drawbacks relating to first and second generations of biomass for biofuel generation. Macroalgae, commonly known as seaweeds, are particularly suitable as substrate for biogas (~ 60% methane) production through anaerobic digestion (AD). However, seaweeds are not yet fully exploited as a feedstock for biogas since some obstacles need to be overcome. In general, prior AD a pretreatment step is necessary; this has to be efficient both in terms of methane yields and energy consumption. An optimal organic substrate concentration has to be selected in order to maximise the yields and avoid overloading phenomena. In Ireland, the influence of algal chemical composition variation according to genera, season and environment is still unexplored. The aim of this research was to investigate and optimise the biogas production from Irish macroalgae when applying a mechanical pretreatment at different harvesting periods. The “Design of Experiment” (DOE) technique was employed as investigation method, since the response of interest (“methane yield”) was affected by several variables which can interact with each other. The advantage with respect to the traditional method of studying the effect of “One Factor at a Time” (OFAT) is that the DOE can detect and quantify the interactions between variables. New knowledge was developed with regards to three different mechanical pretreatments, namely beating, milling and microwave when applied to the Irish macroalgae Laminaria sp. (Laminaria digitata, L. hyperborea, Saccharina latissima, Sacchoriza polyschides and Alaria esculenta), commonly known as kelps, showing that beating achieved a more optimal performance over other methods. New data were generated regarding biogas production from the Irish macroalgae Laminaria sp. and Ascophyllum nodosum, with Laminaria sp. exhibiting 40% more methane. During the seasonal investigation, it emerged that, in terms of the Irish climate, the summer and autumn were the best harvesting periods of Laminaria sp. for methane production. The organic substrate concentration influenced the most the process, with general higher yields at lower concentrations (below 2.5% of Volatile Solids). The impact of this beating pretreatment influenced positively the methane production in autumn (between 50-60%) and at the lowest organic concentration (1% of VS). In terms of pretreatment energy consumption, more energy was generated with respect to the untreated scenario for the material harvested in autumn and winter.