There is Abbyy Lingvo for iOS, from the DSL authors. But unfortunately it does
NOT support user-provided DSL dictionaries yet. Abbyy promised to provide this capability some time in the future and not (yet) delivered it.
In older Lingvo for iOS there was some hackish way to force Lingvo to understand the user-created DSL dictionaries (well, not DSL directly but compiled from DSL binary dictionaries in LSD format). It the latest Linvo 2.3 Abbyy closed that hole
So the only way I am aware at the moment is Stardict dictionaries. There are multiple applications that support Stardict dictionaries on iOS.
And there is a command-line tools to convert DSL to stardict format. They are not easy to come by (but 100% legal) since they are mostly used in Linux world. I've seen those tools custom-built for Windows though. The process is not very easy for non-techies, that's for sure.
We've been asking Konstantin for Goldendict for iOS for quite some time, that would be just awesome, I would buy it at the very second it is released
Without GoldenDict for iOS there is only Stardict route at the moment.