The term “onion architecture” was coined by Jeffry Palermo in a series of blog posts in 2008. The main principles of this architecture metaphor are (1) to use a hierarchy of dependencies, where less fundamental (or central) parts depend on more fundamental parts, but never the other way around, and (2) the most fundamental part is the model, which implements the application's data model in the form of model classes while data storage is a separate and less fundamental part that must not be coupled with the model.
In fact, Palermo and his followers put a lot more into this architecture metaphor, such as using "repository interfaces" and "service interfaces", but this is not really essential for the onion metaphor. Also, they are using a different terminology. When they are using the term "domain model" instead of simply model, they are confusing the term "domain model" with "implementation of data model", which is what model classes do. A data model is derived from an information design model, which may itself be derived from a domain information model. This is the basic development chain in model-based software engineering.
In principle, a Data Mapper approach, if it is not based on a platform-specific ORM (annotation) technology, but rather on some form of platform-independent mapping logic, can be used for storage management in an onion architecture.