The spearmen in Ireland need fixed; they're supposed to have javelins and will be redone. Also, British units are generally low morale. Probably one of the best investments you can make is to recruit Cwmyr once capturing a midland province (there's a southern British equivalent in works currently). Certain units, like Cwmyr, and chariots, greatly improve allied morale.

With chariots, once you've disrupted the line successfully, bring them around behind your soldiers. They raise allied morale so much that your men won't break unless a real slaughter is going on, and chariots also lower enemy morale. Units like Cwmyr can be used to fight with their superior stats, but if placed behind your charging line, act like chariots (but don't lower enemy morale); however, if you let them fire at will, they'll throw their javelins into the enemy, though you risk hurting your own soldiers.

As for cavalry, the Brihentin are a mistake; not supposed to be available in Britain. However, there will be skirmishing cavalry in Britain soon, and there will be cavalry in Ireland to get. There will be continental cavalry they can get too; the best statwise will be in one of the Belgic provinces (a Remi unit that should be in one of the forthcoming builds).

The Casse are an armor light faction, but they've got some good units being worked on. They'll be able to get some axes later on (axes ignore half the armor of an enemy), their skirmishing cavalry, their new chariots aren't in yet, some Gallic spearmen in the southwest, some remade stuff, etc.