Paul, I agree with some of what you say. The Irish drift nets fly right in the face of the notion that fish should be harvested from individually identified, sustainable populations. If you look at the quotas of Irish fish county by county, you will see that Kerry has a disproportionately large share of the total - almost exactly one sixth of the total harvest - yet doesn't itself have any major salmon rivers. Is it just coincidence that Kerry lies at the south-west corner of Ireland, and that all salmon heading for SW England and Wales, as well as the Irish east coast rivers, are likely to concentrate at this point as they turn to swim east? I think not.
Nevertheless, the fact remains that there is a demand for salmon which needs to be fulfilled somehow. That demand has been hugely inflated by the cheap and readily available farmed fish is recent years, and it isn't going to go away. Would you rather support the vile salmon farming industry by buying their third-rate product? Until a way is found of avoiding the hazards to wild fish populations that they represent, I'd rather not give them a penny of my money. Moreover, there are people who prefer to buy wild salmon, and cutting off the supply of legitimately caught fish will only encourage a resurgence in incidents of large scale poaching, which has reduced in recent years.
I cannot see any objection to draft nets in estuaries. Provided that the rivers have a healthy and self-sustaining run of fish, I believe that a harvest can be taken. Take an example from one place I know a bit; the draft nets in the Killary Harbour in Connemara. These nets take fish destined for two rivers, the Bundorragha system (the Delphi fishery) and the Erriff, that share this estuary. Both have healthy numbers of fish, and I believe they genuinely are a sustainable resource, that can afford moderate cropping. It's worth mentioning that Peter Mantle, the proprietor of the Delphi fishery and one of the leading voices in opposing salmon farms in the area, actually sells smoked salmon taken from these nets. Although he has an interest in ensuring the maximum number of fish return to his river, it appears that he thinks this is the 'least worst' way of satisfying the demand for salmon.