Fifteen decades by a tree Cryptically expresses me - A garment worn in times of old, Five letters long, I keep out cold.

“Coat”

While a coat is also a garment worn to keep warm, it does not fit the specific clue of being five letters long. 'Coat' has four letters.

“Shawl”

A shawl is a type of garment that can keep out the cold, but it has five letters; however, it does not connect to the riddle's reference to 'fifteen decades' or its Roman numeral representation.

“Dress”

Although a dress is a type of garment, it doesn't fit the description of keeping out cold as effectively as a cloak, and it doesn't relate to the Roman numeral clue in the riddle.