He didn't necessarily get it wrong - "foreign" names can be transliterated (just re-spelled using Irish spellings), transposed onto similar Irish names, or translated, where the name has a specific meaning in another language that can be translated (this is common with Surnames, such as Smith for Mac Gabhann or Welsh for Breathanach), and sometimes more than one version can be in use. But it's not that common a name, and the few sources that I can find seem to prefer Doiminic.
You're over-interpreting that Wikipedia article. It doesn't say that the Irish Dé was derived from the Latin "dies".
The Latin "dies" and the Irish dé are both derived from the same Indo-European root, the Irish didn't get it from the Latin.