Using Darkwood in place of normal wood will impact the amount of wood that will be affected by the Ironwood spell, making it more likely that larger items can be made at half of the allowed weight by the spell, resulting in a +1 item. Item weight ratios are steel:wood, 6:5. Darkwood is half the weight of normal wood. Metal breastplate weighs 30 lbs, so in Darkwood that's 12.5 lbs. (includes helm and greaves). As the Darkwood spell has a duration measured in days; items, where recorded, should be date-stamped and tracked. The DM may devise ways to modify this limitation -- an example is given later.
Wood Shape specifically mentions that items made will be crude. Forming a breastplate should be speed itself.. engage the spell on a hunk of wood, press it against the chest, smooth it down, pinch off the excess, poke some holes for leather straps.. boom, you're done, 5 minutes if that. Same for helm and greaves. Crafting is so much easier because it is being done directly against the subject. Why the result is crude if given sufficient time should be up to the DM. Imagine if you had a similar spell "Iron Shape", items made by hand would be crude too. But, tools and methods were specifically made to work iron intricately without spells being used. With a spell based malleability conferred on wood, one could imagine tools, presses and moulds devised by the druids that would enable shaping the wood more finely.
Consider that Wood Shape may not even be necessary in getting good base items to apply Ironwood to. Beyond the normal means of crafting wood (by hacking at it with cruel implements of iron), strips of wood and woody vines or from coppiced Darkwood stock could be woven into the shapes of armor and shields. Visualize the beautiful knotwork/weaving of Celtic motifs. Using a Craft Weave could lead to excellent items, at an even lower weight (about 8 lbs I'd say).
A Druids mindset is important to consider while looking at how the spells interact and how they can impact a druidic clan or tribe. While there are many variations of druid-kind in the resource books, its good to consider the core values. The Druid is the source of wisdom, lore and civilization to his group. They are self sufficient. What the common man calls civilization actually blinds a druid in his Commune with Nature, and so the more powerful the Druid, the further he wishes to be from cities and towns. PC druids, of course, can take full advantage of all that is available to him, excepting prohibited items. NPC druids make do with what they have to work with, and may be quite creative in how to meet their objectives.
With this in mind, it seems sad to me that a vibrant NPC druid based group should be so beholden to an outsider wizard to cast a permanency in order to make a +1 ironwood shield by an 11th level druidic caster, that is functionally equivalent to what a fifth level wizard armourer could make out of iron by himself. Let's take a different path and look at the Hallow spell.
Looking at the list of spell effects that are listed to the Hallow spell shows how it can be used with purpose to accentuate an aspect of the clerics or druids power, such as a "room of counsel" with the zone of truth active within. However, much of the usefulness is conferred by the "goodness" of the Hallow (or evil of the Unhallow) that gets lost with a True Neutral Hallow. How could this be addressed? As worded, "Spell effects that may be tied to a hallowed site include...", does not necessarily indicate that the list is exhaustive. In the case of Neutral Druids, Hallow and Unhallow are one sided and not in the spirit of a natural balance. Allowances could be made to allow neutral divine casters different but similarly powered natural hallows.
So, imagine the Druid's holy armoury where the Ironwood spell is the active spell tied into the Hallow. All active Ironwood items that are present at the hallows creation are then linked to that Hallow. During the months that the Hallow is active, new items can be crafted and blessed with the Ironwood spell and so become linked also. To retain the link, and thus continue the Ironwood on the item, any item that leaves the Hallow has to return to the Hallow within the expiration period (11 days for an 11th level Druid) and remain within to renew the link, over the period of arbitrarily an hour, and then may be removed again. When it finally becomes time to renew the Hallow, all Ironwood items must be present or the links on the missing pieces will be lost. If the items fall into enemy hands they will return to simple wood items within a few days. End result, effectively "iron" weapons and armour for the druid and his cohorts as long as they don't stray too far from the holy grove. Not over powered and has great story flavor.