Sounds like it could be wrought iron, which were very individualistic and quirky alloys depending on where and how made. Some types were suitable for making tools and weapons by cold working the metal. Some types were rust resistant, and some types were very, very rust resistant. It all depended on which impurities were in the alloy and how much (copper, tin, etc. Wrought Iron with a high phosphorus content was the most rust resistant.)
I believe that a ulu shaped vegetable chopper made by my great grandfather in the late 19th century is wrought iron. He was not a smith, per se, and probably cold worked a piece of wrought iron he got hold of into the form of a chopper. Superficially, the surface looks somewhat similar to your Celtic piece, especially on first glance. I've marveled at it's perfect condition, the color sort of a deep, very dark brown, a lot like a bronze patina, and without a spec of rust. With wrought iron, pieces centuries apart and found in widely varying environments can both appear to be in somewhat similar condition. Anyway, for what it's worth, that's my theory.