Declan is talking about the Rowan tree which is known for its protective qualities and is also known as the Mountain Ash. You can find it described as such in Greek mythology and elsewhere. I believe Harry Potter’s wand is Rowan wood but it’s been a long time since I’ve read those books.
Just in case you don’t know, The Demon Accords on Vella are background books to John’s Demon Accords series, which is phenominal and starts with “God Touched”
Personally, the only thing I can tell you about Rowan wood is, if it’s the same Moutain Ash that grew in my yard when I was a kid, the red berries would make birds drunk in the fall when they ate them and they flocked to the tree. We’d have them fly into windows and find them unable to get off the ground without having hit anything.