Go as low as you dare.
In September 2008, we offered 50% on a house; they refused. They accepted at 56% of the original asking price.
There were, admittedly, complicating factors involved (house required total modernisation and a huge amount of renovation). However, the fact remains that a respectable company of estate agents had valued the house as it stood at one price, and we got it at quite another.
If you can believe it, we still wonder whether we could've got it for less, ie. closer to our original offer. This keeps my DH awake at night sometimes.
So, as I said, go as low as you dare, so that you don't have any regrets.
Of course, there will be regional variation. Where I live, there is still a high demand for housing (and not much building going on) which is sustaining house prices. Whether this will last remains to be seen. I suspect that for a variety of reasons (strong-ish local "industries", etc.), the housing market here is and will stay reasonably robust.
The Ireland situation is analogous in particular, I think, with certain urban centres of the UK (e.g. Manchester and Birmingham), with the property boom propelled by construction and a false prediction of sustained credit and an ever-increasing market for the properties built - a Ponzi scheme in bricks. At one point, new-build flats in the centre of Birmingham were selling for extraordinary prices; their value has now collapsed utterly.
The point about BetaDad's levelling-out is that, as with all averages, it will hit some regions harder than others. There will be a larger decrease in the market in some areas, while others will see only a minimal decrease. You need to research your area to work out which kind it is.