Hmm, what would I do...
I seem to have a lot of success with the Halronds in Extended Factions.
So tough, skilled light cavalry on fast, hardy hobin horses who would make up the flanks and the scouts. The centre would be screened by a skirmishing line of kerns, tough and fast infantry with javelins who would advance and pelt the enemy with sharp sticks. Short spears and shields give them a bit survivability if surprised by enemy cavalry, but they are still lightly armoured, so after providing that skirmish support they will melt back to the mainline infantry, dominated by muires and galloglasses, providing a backup flanking force if pressed.
Providing overwatch here, the famous Halrond helwyr, raining steel death down from afar with their longbows.
As to terrain, hills, plains, forests, mountains, they can do it all. High ground for the helwyr is preferable, but not necessary as at close range, those bows have enough power to kill a horse and pin the rider to it. An ambush on a forest road with light infantry and the helwyr staying back behind the treeline would be deadly.
Their weaknesses? No real heavy troops, and especially not cavalry-wise. On the other hand, kerns and especially ridires are quite adept at taking out heavy cavalry, so this isn't a big problem. Gallowglasses and muires are as close as they get to heavy infantry too, with brigandine and chain armour. Still, tactics, speed, skill and sheer toughness go a long way.
To Paramythion, I must finish Extended Factions at some stage, and I probably will. And when I do, you shall be the first to know.