A Gaelic Alliance is a storyline quest which is started after Wisdom of the Ancients by talking to Caustantin mac Fergusa. He is negotiating an alliance by marriage with the kingdom of Dal Ratia, which is near completion. He has sent two groups to negotiate with the king, but he has lost contact with them for unknown reasons. Your task to find out what happened and bring Fingualla, the daughter of King of Dal Ratia, to Perth. This will unlock the Doncoirre Homestead on the map.
When you enter the homestead, you find out that the homestead was being attacked and burnt to the ground. When you come across the only building on the cliffside not affected by the attack, you find the few remaining villagers here, including the princess. They will be hostile when greeted, but when you explain that you were sent by Caustantin, they will tell that the homestead was attacked by Vikings with a black and purple sigil, which are the colours of Jelling.
They will tell that they took the king of Dal Riata as a prisoner to beachhead one day from the homestead. They ask to fight the Vikings and bring back Donncoirre.
Once you've entered the new location on the map the first thing you encounter is a conversation between Skakki and some of his pictish followers. Once the dialogue is finished the centries will spot you and will run away to raise the alarm. A hard finesse check lets you take them out or you can let them go by choosing the dialog option "let's exploit the difficult relation between Skakki and his followers". This could make the next fight harder depending on what you do.
When you approach the camp you can choose to talk about the death of Mac Taidg when you have 3 diplomacy or attack straight away. When you choose the diplomacy option two of the Picts will ask for a bribe - and each require 240 valuables (trying to bribe them for '6 pence each' results in one asking for the full share, and the other turning hostile), but this 240 valuables gains morale with cunning followers, but loses morale with greedy ones. Nevertheless, the two Picts will make winning the fight easier. Be carefull because there are many spike traps in this battle. Once the battle is finished there is a boulder near the cliffside of the camp which can be pushed off onto aVviking guard below.
All that remains now is to go down to the beach and engage in combat whith Skakki and his men. Combat will start immediaetly and bringing more than one archer will make the fight easier because the starting distance between the two groups is large.
Skakki will automatically escape at the end of combat and the only thing left for you to do is head into the cave and free king Donncoire.
Thralls or free men?
There are three people standing opposite of the camp, which turn out to be thralls caputured by Skakki somewhere from a fishing village allied to the Picts.
You can capture them or choose to set them free. Setting them free will lose morale with greedy followers, but with gain morale with altruistic ones. Also, your reputation with the Picts will increase by 5 and your reputation with the Gaels by 10.
An unbreakable shield called "The Valkyrie's Flower" can be found in a chest at the edge of the beach when you turn right when going down to the beach after clearing out the camp where the boulder can be pushed down.