Today I conducted my second successful solo DH run, so I think I can offer some fresh (and by "fresh" I mean "noob") advice:
1) Greater Confusion Pots are your best friend: prepare a macro to use one on yourself and, if you find yourself in a difficult situation, you can fire it and run - they are usable with 2-handed weapons, and are dirt cheap. Remember there's a timer, though (30 seconds, I think), so they are best complemented by honor. Which brings me to...
2) Make sure you are at least Follower of Honor: you really should train your skills anyway, so just honor everything you are about to kill (Miasma and the minotaurs are great in your range). The reason why I suggest this is that...
3) Embracing Honor (ie using it on yourself) makes life a lot easier: while you can definitely collect all the keys without it, it's easier if you can separate the nameds whose keys you need, and embracing honor allows you to do it without too much danger.
4) Bring potions and apples: I was not accustomed to using either, but today I realized how easier it's your life if you do. Greater Strength and Agility pots are very effective in counteracting Dreadhorn's perennial not-quite-curse; Total Refreshment can make the difference between life and death (especially if you can't/won't use Divine Fury). Apples, while not life-saving, are certainly worth their weight, since they allow you to swing at full speed and full strength for at least a while.
I also used the Grapes of Wrath, but I'm not conviced they made any noticeable difference.
5) Know your weapons: As it's already been posted, Irk and her sisters are not really killable without a 100% elemental weapon: the little mana you will be able to drain is best used for Lightning Strike. Remember what you're doing, though - Dreadhorn is susceptible to physical attacks instead, and if you don't switch weapons you are going to be in a rough spot, as my first UNsuccessful run proved (I was still holding Soul Seeker and didn't realize it before finding myself out of mana and next to dead - then dead).
Furthermore, using the weapon's specials can be extremely effective, but I'm not sure that is always the case: the second UNsuccessful run went awry due to excessive reliance on a Leafblade's specials, which drained my mana at the wrong time.
6) Do not despair: DH's "Wave of hopelessness" notwithstanding, even if you die there might still be a chance. The two copies of the "final" key (Essence of Wind) you have left after entering don't expire for a few minutes, depending on the time it took you to actually enter, and they can be used as soon as you enter Ilshenar - today, a couple minutes after entering, my dragon's barding broke. I saw I still had 300secs left, so I took the boat, recalled to the bank, rebarded, recalled to a moongate, returned to spirituality, returned to the Weald and used the second Essence of Wind - I also managed to deal a few "free" hits on Dreadhorn, who was apparently astonished at my chutzpah.
7) Be prepared: the more you plan ahead, the easier it gets. Today's success was the result of taking note of all I did wrong in the previous runs, and trying to prepare for it. So I ended up with 10 each of Confusion, Agility, Strength and Refresh pots, and 20 GH (I ended up using between 5 and 8 of each); I ended up using Mace Fighting because it has the only fast elemental damage only weapon (Wind's Edge) for Irk together with a heavy hitting 1-hander for DH (Diamond Mace); I ended up fighting Irk near the wall so, if I died, she'd be somewhat busy with the Pixies on the other side when I returned, etc.
Hm. Ok, I guess that pretty much sums it up =D