I guess you got this sentence as a fill-the-blank exercise and the word eileanan was missing? If so, I don’t think there is any context that would make it obvious you need to choose the plural word.
I can only guess that the other options to choose from were unacceptable for other reasons (like eg. not being in the nominative case needed here), but as I don’t know exactly what options you did get, I cannot be sure. If there were more than 1 options grammatically acceptable, the exercise seems to be badly designed and needs reporting.