Translation:It is two hundred kilometers from Dublin to Cork.
Idir is "between".
The NEID has a couple of examples for "miles between":
tá 10 míle idir seo agus Cathair na Mart - "it's 10 miles to Westport" (literally there are 10 miles between here and Westport) tá turas céad míle idir seo agus Corcaigh - "it's a hundred miles from here to Cork" (literally there is a journey of 100 miles between here and Cork)
It also suggests tá céad míle as seo go Corcaigh (literally there are 100 miles from here to Cork).
But in this case, where there's a fairly straightforward translation, without any idiomatic issues, I don't think that "there are two hundred biles between Dublin and Cork" is a good translation of this particular exercise.