The Picts

[Peasant Phill]

The first thing you do as the Picts, apart from allying with the Vikings, is attack the scots at their production center (Al Riad or something similar). Be sure to take at least one crossbow unit so the scots have to charge you and your army in a prepared position. Once the Scots lose their ability to produce units (especially their dreaded clansmen) they live on borrowed time. Next attack the province the Scottish king resides in and a hefty ransom is yours for the taking (Pictish cav will be very useful for capturing the routed king). Do this twice and your money problems are over for a lengthy time.
Take some time to consolidate your new excellent position. If you've done it right, you now have two advanced production centers, an experienced army, a nice amount of florins and a easily protected 2 province border to the south.

your next move: Ireland
You can also take the island between Ireland and the British mainland if it is political possible. In my case, the Vikings held it so I held of until a more appropriate time.
If the Scots still survived in Ulster, you might want to kill them off first as they could benefit from your hard work. After that, again attack the Irish production. It's the middle eastern province. Attack with as much men as you can safely muster and don't hold back on the mercenaries as you want to take the province in tact. Like the scots, the Irish are hamstringed after this easy win. You might want to take to the offence with your mercenaries alone (and maybe a decent general) as you want to hold your foothold on the Island but want to use your mercenaries productively. It's questionable you can win a battle with them, but at least you can damage the Irish troops somewhat, damage they can't repair for some time.
When you strenghtened your hold on your Irish province(s) somewhat it's time to go back on the offensive. If possible try to isolate the king again for a good ransom or if this isn't possible, try to drive them towards the rebel province. The Irish will have to fight a big rebel force and won't come out unscatted. Now it is time to move in for the kill. The Irish last stand won't be an easy battle but handle it wisely and you can't lose.

Hints when facing the Irish in battle:
the Irish forces will have a lot of skirmishers (bonnachts, dartmen and kerns). they'll probably be half of the army your facing, with other half being royal bodyguards, gallowglasses and the occasional (armoured) spearmen. They can either be (1) aggressive and charge you with their melee troops (believe me taking a charge from a wave of gallowglasses and royal bodyguards will result in a lot of casualties on your side) and leave their skirmishers to hang back to pelt your troops when the fight thickens or (2) they will first try to thin your ranks and disorganize you with their skirmishers and charge in later with the melee men.

(1) keep your men in tight formation to absorb the charge and flank with your celtic warriors and beserkers (if you got any) after the charge evolves in a prolonged fight. Send your cav (pictish cav will have a field day) around the fighting to attack the unprotected skirmishers.

(2) keep your men in formation with your crossbow men at the front. Don't chace after them. It is questionable the kerns, bonnachts and dartmen even get to throw their missiles before they turn back out of range. Eventually the melee troops will engage and you can follow (1).

So what should you remember when facing the Irish as the Picts:
* bring along a decent cavalry force to deal with the skirmishers
* bring along your best spearmen to absorb the enivitable and crushing charge
* bring along crossbowmen to repel skirmishers in the first moments when they are still protected by the Irish melee units
* always keep your infantry together before the melee starts
* don't chace their skirmishers with anything other than cavalry or you could be in for a nasty surprise