**1. Introduction**

The European Union's (EU) climate and energy package sets binding greenhouse gas (GHG) emission reduction targets for all EU states by 2020; these include a 20% cut in GHG emissions, to produce 20% of energy consumed from renewable sources, and a 20% improvement in energy efficiency [1]. The Republic of Ireland, in particular, has struggled to meet its emission targets, with most recent estimates projecting a 14–15% shortfall, resulting in the country projected to pay up to €103 million in carbon credits to compensate for its lack of climate action [2,3]. Ireland's agriculture sector has consistently remained the single largest contributor, accounting for 33% of all GHG emissions in 2017, and 46% of all non-emission trading system (ETS) GHG emissions [4]. The country now faces a dilemma, to either limit or reduce the growth of its agriculture sector (which is vital to Ireland's economy) or to disregard its environmental obligations.

A promising technology with the capacity to provide both renewable energy and GHG reduction, particularly in the agriculture sector, is anaerobic digestion (AD). AD is a natural process in which microorganisms (hydrolytic, fermentative, acetogenic and methanogenic bacteria) break down biodegradable material in the absence of oxygen, producing biogas (a mixture mainly composed of methane and carbon dioxide). These systems are beneficial for improving on-site energy generation, upgrading wastes, and producing a nutrient-rich fertiliser from the digester effluents. They can also reduce pathogenic loads, odours and greenhouse gas emissions emanating from the agricultural processes [5–8]. Furthermore, the technology has received considerable research attention, advancing its potential capability through optimisation strategies [9–14]. Despite the apparent benefits, Ireland has been slow to adopt the technology, ranking 20th in AD penetration among the EU-28 countries [15–17]. A contributing factor to the low deployment is the concentration of "large scale plants", particularly in Europe, where the siting of such centralised facilities has been based on the availability of significant quantities of biomass feedstock [18]. However, the biomass quantities in many Irish farms are currently insufficient to meet the feedstock requirements of medium-and large-scale AD plants. The situation is worsened when considering that the average dairy herd in Ireland only consists of approximately 90 cows in 2018 [19].

The application of small-scale anaerobic digestion (SSAD) plants with an electrical output of 15–100 kWe, holds promise in overcoming the technical and economic barriers associated with treating lesser biomass quantities [18]. SSAD may be particularly useful for the Irish dairy industry, where there is a large livestock population (1.4 million dairy cows) [20], there are predictable process energy demands and reliable feedstock collection potential, its deployment is promising. Despite the potential of this technology, previous studies have largely focused on the implications of deploying medium to large scale AD plants (>100 kWe) with relatively little focus on the Irish context [21,22]. Therefore, a lack of understanding is apparent in the applicability of SSAD plants in stand-alone agricultural environments within Ireland [23].

The goal of this study was to provide an initial assessment of the viability of SSAD on commercial Irish dairy farms. Thus, not only benefiting the reported case study but also other countries and regions, especially those with significant agricultural and livestock productivities. To achieve this goal, the following objectives were put forward:

