Regarding added liquid: one of the reasons it can vary so much is storage method and local humidity. We store our flour in plastic bins and in a very dry part of the house so I always need to add more liquid to recipes. Another reason is measuring by volume - flour can be compressed pretty easily (by the weight of other flour or tapping flour into a measuring cup) so volume measuring can be pretty shifty - a kitchen scale can really help in that regard.

I'm not an expert on Soda Bread specifically, but am on bread. I looked up the source recipe, and it seems way off to me. The hydration with this much dry ingredient to buttermilk is way low, and is certainly going to make the bread crumble apart as you described.

Less than 50% hydration is more akin to pastry than bread, and I'm not even aware of any standard loaf recipes that go below 50% because it will just crumble apart.

The other thing is the addition of germ, and the way the recipe just says to mix everything together. Absolutely wrong. If you're not blooming or otherwise hydrating germ separately, and just throwing it in with flower, it's going to give a really weird and inconsistent texture with rough patches throughout the loaf. It's also weird they don't discuss kneading or resting for hydration because of the germ, but that's a different topic.

At a minimum, you should try adding an egg to bind things together. I would also be weary of any online recipes that don't measure by weight for baking. They're almost certainly not coming out like the pictures they show.

https://www.odlums.ie/?s=soda+

Here's a selection of soda bread recipes from an Irish baking company. I can't remember if I've tried any of these specific ones but I have made a few of their other recipes before and they were good.

As a daily consumer of brown soda bread in Ireland I feel obliged to say that recipe is not the best. Never ever have I seen sugar (it's just not really a thing in bread here) and with buttermilk there's no need for baking powder, only baking soda.

The Odlums recipe linked by another poster here is solid. Generally wouldn't bother with wheatgerm or bran myself unless I had it to hand and I generally don't though I have added it occasionally in the past. Decent option if you're looking for extra fibre though.

Edit: on buttermilk I would generally add until it's semi sloppy. Kinda do it by feel at this stage but I do recall that Odlums recipe being a good start before. You may need more if you're adding in the bran / germ as I think they soak up quite a bit.