I've only just found this little thread, while looking for something else.

From the context here, I would disagree with Malcolm that this is just a textual corruption. Looks more to me like a local variant where the singer embellished the description of the coffin to emphasise how grand it should be.

Deal forms a small tree, so its planks will always be narrow. The wood is very white, with a fine even grain. It was used, among other things, for the decking of very high class ships. A good example of the use of a similar descriptive phrase is in Captain Frederick Marryat's "The Three Cutters" (Chapter One — Cutter the First).

"The deck, you observe, is of narrow deal planks as white as snow; the guns are of polished brass; the bitts and binnacles of mahogany: she is painted with taste; and all the mouldings are gilded."

Nails were quite scarce in early times. Until well into the industrial revolution, they were made by hand. For larger items, including almost all furniture prior to the availability of cheap manufactured nails, wooden dowels (trenails) were used. Screws would come much later. Making a coffin using (scarce) nails would indicate that it was a fine object and relatively costly. Trenails are still used in timber framed houses and (usually called dowels in this context) in furniture.

:-)