It's not a type of bread I've made much, but I recall reading a lot of complaints about other people with the same problem doing similar recipes. The first thing I'd do is take a look at Sodabread Info, then investigating the boards at The Fresh Loaf for similar breads. Since the subject has been extensively blogged you might try a Google search using the terms Irish, soda, bread, and Guinness; and looking for people who talk about getting the right degree of sturdiness. Then there are the usual food sites.
At it's most cohesive, traditional soda breads have a sort of scone like texture, and you're not going to get more slice integrity than that unless you develop the glutens.
If you really are using Odlums, you should know that it's very, very soft and doesn't have much in the way of gluten to develop, no matter what you do. The first change I suggest trying is switching from Odlums to either a good AP flour (maybe even King Arthur AP, which is very hard as APs go) or a mix of 3/4 AP with 1/4 whole wheat, or even 50/50. Harder flour may do enough for you to solve your problem... Or not.
In the traditional sense the more whole wheat flour, the more "brown" the bread, but the more whole wheat the more other issues you raise. If you want "brown," you might think about replacing 100ml of the the Guinness in your recipe with a three tbs of molasses. As it happens, molasses enhances Guinness hugely.
Another first adjustment is to cut down the beer to around 275ml - 300ml. You want a dough you can work at least slightly. Your recipe calls for nearly 100% hydration, you might want to cut that down to 75% ish. Wet and sticky are fine, as long as the dough is at least somewhat workable.
If you're open to a chewy, dark, yeast bread -- sort of like pumpernickel, but without rye and with a bit of Guinness flavor you can use one of the several on the Fresh Loaf, I can teach you how to improvise your own recipe, or I could even write one for you. Once you know the basic ratios and techniques, good crust and crumb are easy. Creating your own bread becomes mostly a matter of nailing down the flavor profile -- which may or may not be quite as simple. But, if you don't have a good grasp of the basics like pre-ferments, "touch," kneading, multiple rise, and formation -- those are things you have to learn first.
Edited by boar_d_laze - 6/7/12 at 10:58am