I don't have the information from my previous homebrew handy, which had a lot of "thou shalt/shalt nots" in it. I do however, have a paladin's code from my most recent psuedo-celtic homebrew, which has some thou shalts and shalt nots.
Fionn's Code
1. Let thy sword be the first to be drawn and the last to be sheathed. Let thy death be noble and glorious; die not in vain.
2. If thy enemy and thine own son come before ye to be judged, let thy judgment be fair.
3. Leave no man unfed who has mouth with which to eat.
4. Make no promise at night that ye cannot fulfill on the morrow, and no promise during the day that ye cannot fulfill at night.
5. Leave no curse unanswered.
6. Allow no harm to come to the stewards of life.
7. Leave not the fruit to spoil or the wine to sour.
8. Be quiet in peace and angry in battle.
Practically, the code means that the paladin must exemplify the following traits:
Valor
A paladin must be the first onto the field and the last to leave. He must be brave in the face of adversity but should not sacrifice his life unnecessarily. Retreat is not prohibited, but a paladin should not leave his companions behind. He is a leader of men and women, and the safety of those he leads is paramount.
Justice
A paladin should be fair in dispensing justice. He should decide conflicts based on the facts, and not allow personal preferences to color his judgment.
Generosity
A paladin should be generous to a fault. He should strive to feed and clothe as many people as possible. He may do this through any means that don't violate any of his other strictures. For example, he may donate adventuring loot, or he may be a generous employer.
Dependability
A paladin's word is his bond. He is as bound by the promises he makes as he is by a sidhe's geas.
Vengeance
A paladin should never allow a wrong to endure. He should endeavor to bring justice to the harmed, and protect the innocent.
Protection
A paladin should protect those that produce new life. Without mothers and farmers, the great cycle of nature would cease. A paladin should never kill an animal that is a mother to other animals.
Merriment
Nature's bounty is a gift, and should not be wasted. Food, wine, sex, music, poetry, and all of the other gifts of life should be enjoyed to their fullest.
Temperance
Above all, a paladin should know how to apply himself in every situation. He does not fight his friends, love his enemies, insult his clan, or spill his wine.