By this period, the Irish and Scots were hugely different; the Irish were an amalgam of mostly Gaelic and Nordic culture with Norman influences, and the Scots were a mash of Gaelic, Nordic, Pictish, Norman, Saxon, and Angle influences. They didn't think of themselves as the same people by this time; it's pure fantasy to think such a confederacy would ever occur by this period. The last time the Irish and Scots thought of themselves as remotely similar was when Robert the Bruce sent a letter to the Ui Neill telling him that 'our nation' was in trouble; however, the Ui Neill snubbed him anyway, feeling that 'our nation' was an incredibly rude statement, because the Ui Neill did not feel they were part of the Scots (nor did any of the other Irish). They only accepted aide from the Bruce because of threat of invasion from England.

Irish and Scottish armies were composed of different types of soldiers, they wore different types of clothing, they spoke (and speak) two different languages, etc. The only thing they have in common is a common ancestor. This is like saying the English should be confederated with Germans. Many of them had the same ancient ancestors, so why shouldn't they be a confederation?