Before the vikings came the Irish armies were almost completely made up of lightly armed kerns with lots of darts and javelins they used to weaken the enemy. Only the king's bodyguard would have good armour and would be mounted. I don't know much else about 7th century Irish.

Either make their skirmishers really good at melee or their spearmen really good at skirmishing and really fast on their feet.

only the Dal riata had bows as far as I know.