You have to find one that fits. You can't bend the wrist to adjust fit, as there is a steel rod running the length of it. If you are in Dublin, I'd expect you have access to 2 1/2" ammunition, more so than most Americans, a good thing, as I'd also expect most of the guns you see will be 2 1/2" chamber. Typically, a Darne will feature heavy proof, and barrels, not always, but far more often than not. Since the guns tend toward being light in weight, you will find, usually, that the gun is capable of digesting heavier rounds than you can stand to use. This is actually a good thing. Many will be tightly choked, this is usually not a good thing. A lot of them have sling swivels installed, something most Americans find outright offensive. I'm not in that group, because my hunting is rough hunting, about as rough as it comes. There are two basic designs, small key, as in Halifax, or R model, and large key, as in P or V model. There is no parts interchange between the two models, they have nothing in common except that the breech slides. The large key guns cost more money, and have better trigger pulls, out of the box. The small key guns are, of course, more common.
The advice to shoulder and, if possible, shoot one, is very well taken. If the buttstock is the least bit loose, do not fire it-this is good advice with any gun, actually, but, a Darne will break the stock if used in this condition, sooner, rather than later.
If you get past the admiration stage, don't be afraid to ask a few more questions, here, before plunking down your money. I'm sure I've forgotten something.
Full disclosure, I imported them to the western side of planet earth for over a decade, and didn't deal in used guns, unless they fit me. I have actually lost count of how many I've owned, and have five guns at the moment, of both types.

I love mine. NOT to the exclusion of other types of shotguns. I have known people in that camp, however.

Never owned or driven a Saab, and my drinking PBR from the can is about as non-conformist a thing as I can come up with.

Good luck in your quest.


Best,
Ted