Grass-fed doesn’t really scale and entails a number of other environmental issues from higher methane to higher deforestation. Even for Ireland in particular, it’s got quite a number of issues

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Increased methane emissions

Grass-fed production requires longer growing times leading to more lifetime methane emissions overall. It also requires more cattle overall due to lower slaughter weight

Taken together, an exclusively grass-fed beef cattle herd would raise the United States’ total methane emissions by approximately 8%.

https://iopscience.iop.org/article/10.1088/1748-9326/aad401/pdf

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Not enough land to meet demand even if 100% of grassland was used

We model a nationwide transition [in the US] from grain- to grass-finishing systems using demographics of present-day beef cattle. In order to produce the same quantity of beef as the present-day system, we find that a nationwide shift to exclusively grass-fed beef would require increasing the national cattle herd from 77 to 100 million cattle, an increase of 30%. We also find that the current pastureland grass resource can support only 27% of the current beef supply (27 million cattle), an amount 30% smaller than prior estimates

[…]

If beef consumption is not reduced and is instead satisfied by greater imports of grass-fed beef, a switch to purely grass-fed systems would likely result in higher environmental costs, including higher overall methane emissions. Thus, only reductions in beef consumption can guarantee reductions in the environmental impact of US food systems.

https://iopscience.iop.org/article/10.1088/1748-9326/aad401

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Problems in countries that have tried to scale it up

New Zealand has tried to scale up it’s grass-fed production and often touts it. To do so, they end up using heavy amounts of fertilizer in their production so much so that some regions need a 12-fold reduction in their dairy industry size just to have their water meet safety thresholds

The large footprint for milk in Canterbury indicates just how far the capacity of the environment has been overshot. To maintain that level of production and have healthy water would require either 12 times more rainfall in the region or a 12-fold reduction in cows. […] The “grass-fed” marketing line overlooks the huge amounts of fossil-fuel-derived fertiliser used to make the extra grass that supports New Zealand’s very high animal stock rates.

https://theconversation.com/11-000-litres-of-water-to-make-one-litre-of-milk-new-questions-about-the-freshwater-impact-of-nz-dairy-farming-183806

Keep in mind that this is the case with New Zealand still using plenty of feed because their definition of grass-fed still allows for plenty of supplemental grain. A fully grass-fed system would fair even worse in that regard

The national dairy industry [in New Zealand] is consistently the country’s largest consumer of grain and feed at approximately 75 percent (Figure 4). The majority of dairy farms are on non-irrigated pasture-based systems (75 to 80 percent), where up to 25 percent of the annual diet could consist of supplemental feeding. With the recently high dairy prices experienced of over NZ$9.30 (US$6.05) in the last two years (Appendix 2), farmers have looked to maximize milk yields by utilizing more “purchased” feed for conversion to milk solids

(emphasis mine)

https://apps.fas.usda.gov/newgainapi/api/Report/DownloadReportByFileName?fileName=New Zealand Grain and Feed Market Situation_Wellington_New Zealand_NZ2023-0003.pdf

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Problems with grass-fed production in Ireland

In the UK and Ireland, the land that grass-fed cows are on is primarily actually not natural grass-land - its natural state is temperate rainforest

Most of the UK and Ireland’s grass-fed cows and sheep are on land that might otherwise be temperate rainforest – arable crops tend to prefer drier conditions. However, even if there were no livestock grazing in the rainforest zone – and these areas were threatened by other crops instead – livestock would still pose an indirect threat due to their huge land footprint […] Furthermore, most British grass-fed cows are still fed crops on top of their staple grass

https://theconversation.com/livestock-grazing-is-preventing-the-return-of-rainforests-to-the-uk-and-ireland-198014