you're the Picts, you've crossbows, shoot em! mass xbow firing before you unleash celtic warriors to the front and cavalry round the back, skirmish and whittle rather than going toe to toe.
also hire mercs, like merc huscarles, or anything with javelins. if you front huscarles with decent merc stuff, you can hold them till you get round the back, but be careful fielding hire viking types as they will suffer from the armour piercing bonus. expect, as the guide says, heavy casualties on some occasions, but they'll be replacing expensive huscarles and you'll hopefully only be replacing cheapo celtic warriors and stuff. i have to admit that my last Pictish campaign bogged down against Mercia, but then i didn't have Ireland.