Well, to the extent that Northumbria warred with Dal Riata, indeed they did. Now, the Picts and Britons were the more immediate and frequent foes, for Northumbria and its constituent/predecessor kingdoms. But as far as the Anglo-Saxon military interaction with the Gaels in Britain goes, it was indeed significant and consequential, by the best account we have of this period (i.e., Bede's).

As for the strengths and weaknesses of these armies, this strays too far into the realm of conjecture, I fear. Anything which might be said would simply be very broad historical/ethnographic generalisation based on the archaeological record, which might or might not be relevant to whatever lesser or greater extent to the (almost completely unknowable) case in point.