How to fix a tap to a big barrel (but it's not a 'water butt' tap)?

I assume you want this below the water level (and thus sealed)?

I assume it is a plastic barrel, is it open topped or if not is there a reasonable sized access hole in the top?

Basically your Google search term is Bulkhead fitting.

Well, at the surface, but it's just a mechanical fix I want really, it doesn't *have* to be watertught.

Yes, it has a huge removable lid so access to inside is easy.

You want a bib tap plus a back nut. And a washer.

Not answering your query, but from experience with plastic water butts, fatigue cracks develop around the purpose-made tap attachment point after a few years. I imagine this would be worse on thin sections of the wall not intended to have a tap. I would suggest that you try and reinforce the butt wall in the area you intend to fit the tap to try and reduce fatigue cracking.

... but doesn't that just fix the tap onto the barrel, how do I feed the water from the 22mm pipe into the tap? ... and aren't most/many of those taper threads on the back of the tap?

If I could find a way of simply feeding the pipe securely through the wall of the barrel that would probably be sufficient but the prices for 22mm straight bulkhead fittings seem exorbitant. About the cheapest I can find is:-

Almost £25 including carriage.

I'm not sure that I fully understand what it is that you want (too long in The Malt Shovel Tavern is my excuse), but something like:

Or, possibly, an IP66/67 25mm electrical grommety thingy might compress down enough to do it.

OK then, just feed the pipe through a clearance hole, secure with a cable tie inside and out, and cover both sides with a fillet of silicone or mastic. If you think you are going to apply large forces to the tap, get the type of tap which is designed to be screwed to a wall, make up a wooden saddle, fix it to the barrel with screws and washers from inside, and screw the tap to the saddle.

No, that's a tank connector for taking water out of the tank by gravity, doesn't do what I need at all. I want to connect a pipe

Well use a tank connector or just an ordinary tap connector.

OK, so the cable gland method should do it. CPC has a useful filter for min/max diameters. One of these should seal to the tank and to the pipe:

Yes, you're right, a cable gland is a reasonable solution, thanks.