All hearing aid companies try to keep their hearing aids as hypoallergenic as possible whether the instruments are in the ear or behind the ear. More information is needed re:where the irritation is occurring (inside the ear canal, behind the ear, at the top of the ear, etc.) If you are experiencing irritation, there could be other causes. Is the area in which you live humid like it is here? That could mean extra moisture getting in the ear canals and causing issues. Or is there a possible underlying issue that didn't present until the instruments were worn?
If it is a behind the ear model, and the irritation is inside the ear, a more open fit may give you relief (if the hearing loss allows it). If it is an in the ear model, the shell of the hearing aid itself can be made with clear material to cut down on any dyes that could potentially cause problems for extra sensitive skin. The best course of action is to see your provider to determine the source and cause, and to determine if a referral is needed to an ear, nose, and throat specialist or possibly to a dermatologist.
Like others have suggested, most hearing aid companies use shells, molds and domes that are all hypoallergenic. In rare cases, when a person still shows sensitivity, the type of material of the mold could be changed. Westone makes some specialty materials that are antibacterial and extremely hypoallergenic for behind the ear hearing aids. I have also had success in this regard by changing the dome to that of another manufacturer if one particular one causes irritation. I have coated receiver wires and acrylic shells with hypoallergenic clear nailpolish and let it dry and that also helped.