I am not familiar with Mountain Ash Berries. Here is the ASPCA toxic plant link:
http://www.aspca.org/site/PageServer..._toxicplants#A
I have also had good luck with local human poison control hotlines. Usually the operators are glad to help and can take human/baby toxic guidelines to help with dogs. Sometimes there are plants that are toxic to dogs and not to humans though - common examples are grapes, onions and garlic.
The best course would be the ASPCA Toxin Control Hotline however they will charge you for the consult. Usually around $60.00 Their phone number is 888-426-4435. Once you set up a case number, if the plant is toxic, then your veterinarian can call them after for no additional charge and discuss specific treatment recommendations for your pet.
Hope this helps.