I was wondering about that. Cattle in Britain are also predominately grass fed and Ireland has a similar climate and environment and a much lower population density and a lot more land for cattle.
Ireland also exports a lot of that grass fed beef, so could presumably export less, and consume more of it to replace whatever it could not import.
A lot of other countries are also be both importers and exporters of food. The problem might be that in some places the quality and range of diet might decline.
In Ireland (and I believe it's similar in GB), beef cattle are usually finished on some mix of silage and concentrated feed with a significant maize component to promote better fat distribution.
There's a continuum between 'extensive' and 'intensive' finishing methods - the former takes longer and uses more forage & grass, and is best suited to native breeds. The latter uses more silage & concentrate, and is used for 'continental' breeds.
Dairy cows will also have pelleted additives over the winter, making up to about a quarter of their intake (largely depending on silage quality). But those tend to be mixes of yeast, fats, and digestible fibre so shouldn't necessarily require imports.