Well Malc, that's a $64,000.00 question. If someone could come up with a single answer they'd get rich.
In Ireland, and elsewhere, I suspect, they use a great big plastic Pike float, a size 6 or 4 bait holder hook. The method is to use the float as a slider, so that you can adjust the depth, and3 or 4 lobworms on the hook, threaded up the line, so that they hang down like Octopus tentacles. You cast this into the line above where you think the fish are lying and rather than trot the float down, they actually walk it down the swim, trying actually to keep slightly in front (downstream) of it. The idea is that it imparts little or no drag. If a Salmon takes the worms, this is usually indicated by several sharp dips of the float, before it disappears down the plughole. To strike, because there may be slack line, the Irish tend to move backwards, away from the river edge, fairly sharply and winding in line and striking all at the same time.
That's one method. There are probably dozens of others, not least the one mentioned by Adrian.
