I agree 100% that Eldorado is a very poor choice for an Irish Stout. While Magnum is pretty neutral and should be fine for a bittering addition, 1 oz. at the levels of AA I have seen in the last couple of years (~ 12%) is too much for the style. You shouldn't get astringency from the steeping grains because you hit 170 unless the pH was too high. Also, you actually should really use distilled or RO water for extract brewing. The necessary minerals were added to the mash used to produce the extract. I would expect some age to mellow it out. If it doesn't mellow enough for your taste you might try stirring a pinch of calcium chloride in a pint.