This is a site containing reports from various brigades of the Irish Brigade during the American Civil War. I can confirm from comparing these reports with those found from the US Army museum site that these are correct and genuine transcriptions of the actual reports. These reports give a good synopsis of individual brigade movements and actions during the units various engagements, and it even contain enough to put together an analysis of military hierarchy by reading reports from various ranks.
Bad Source: While I do not have one to link, a bad primary source would generally be hard to find. Most sources from the era they are from provide some sort of historical value as they can used to study the time period or some forms of satire. My guess for what would be bad is something in poor condition or something so satirical it isn’t clear what it is describing.