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.
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.
It seems like both of these are true: "Cattle are predominantly grass-fed" - yes, but this is seasonal; when they're eating something other than grass, it's an import.