THanks for posting this translation.
"Le eachaibh luath is iuchair throm" means "with fast horses and a heavy key" - "eachaibh" is dative plural, it can't be singular so definitely more than one horse.
"a Sasainn" mean "from England" not "from the south".
"An Gaidheal 'sa leabaidh" means "The Gael in bed" (no verb there) not "The Gael has gone to bed", but that doesn't really matter as to be in bed he must have gone there.
I think the handling of the chorus is inaccurate, but I could be wrong. Gaelic has plenty of absolute constructions, so that finite verbs are sometime avoided, and the chorus is a good example of this. In fact the Gaelic version of the chorus doesn't contain any verbs at all - I guess whoever did this translation thought that verbs have to be inserted to make sense in English (what an inflexible language English is for some people) but I think most of the verbs in the English translation of the chorus can easily be left out with just a little care. But that isn't important, as long as the right meaning is conveyed. What concerns me is that the translation of the chorus seems to me to give the wrong impression: I reckon the chorus is a call for action, rather than a complaint that action is difficult. The idea is not that rising from slumber will be difficult but rather that rising from slumber is essential to retain our culture and not allow the English to wipe it out. "éiginn" means "neccessity" or "crisis" or "distress". A hundred and ten years ago (when Dwelly was compiling his dictionary of mostly 19th century Gaelic) , it could mean "difficulty", but this song was written much more recently that that - Runrig wasn't formed until about 40 years ago, and I think this song is less than 20 years old (but I wasn't run-rig fan way back when, so it could have been written a couple of decades before I noticed it). Of course it's possible that the younger MacDonald decided to use the word in its older sense rather than in its modern one but if so I don't understand why he wrote "le éiginn" instead of "air éiginn", because the old style used the preposition "air" when the meaning "difficulty" was intended and modern dictionaries still indicate that usage.