Important to note, Eilean Donan was bombarded and almost heavily shelled in 1719 by British Government forces after the castle was garrisoned by rebel Jacobian and Spainish forces. After it was captured tt was then blown up, almost completely reducing the castle to rubble.
The reconsturction we see now was done from around 1919-1932. The building work didn't really follow any basis in historical record except for roughly following the layut of the foundations and as a result it's not a faithful or historically accurately e reconstruction. This isn't to dimish it in any way, it's just not maybe as 'old' as a lot of people belive when they see it!
I can't reccomend the book, The Mediaeval Castles of Skye and Lochalsh, enough. It's an amazing record of all the castles around the area here and it's definatley the most definiative resource for understanding them.