Does using Lachlainn as a surname sound strange or could it be okay?

When writing a story you, the writer may develop the characters any way they choose and use any naming process they like. The Books and Authors category might be a better place to ask your question than in the genealogy category, as genealogy has almost nothing to do with the writing of fictional stories. I say almost because some genealogies are total works of fiction, but the is not what genealogy is about.