In terms of space, as others have pointed out, a radius of less than a fraction over two feet is not really do-able in 00. many of us started off with layouts of 6ft x 4ft (maybe you did too?) and this allows about the smallest feasible layout with a continuous loop. the stations possible on the two "long" sides will allow little more than a platform which holds three bogies, or a run round which allows a loco to get around two. If, on the other hand, you have a "U" shaped setup, while a continuous loop isn't feasible, an end-to-end is, and this will give you about4ft to work with at either side of the layout. That type of thing is suitable for a shunting type of thing.
You could have one side as a fiddle yard, and the other a more accurate representation of the goods yard. The "through road" would end as a blank end in an overbridge. Alternately, each side could be a "terminus", so here's a possible idea: One side is the markets branch terminus, the other is the main good yard.
Assembled in modular form, with proper design (possibly via Baseboard Dave of this planet) it should be possible to make it portable.
A couple of "UG"s and perhaps an ex-NCC "Jeep", or a dirty "S" clas in UTA livery would be all you'd need, along with (judging by the space you say you have) maybe a total of thirty or so wagons. I gather you're not that interested in passenger workings, but two old wooden GNR bogies and perhaps a K15 would add a bit of interest. The existing steam engines would be compatible, and their presence in a good area could be passed off as an IRRS special or Sunday school excursion, having stock stored there overnight.
If you can, a circuit is better. This allows the Omagh - Derry local, or an Enniskillen - Derry train to pass through, with just those 2 or 3 carriages. Equally, a 2-coach AEC set with one of them in tow, or in the middle, would add interest as it passed by en route to Great Victoria Street.
You could have a man in a long grey trench coat and a battered leather briefcase on the platform staring at a rail joint; this would be jhb171senior on his bi-annual inspection of the track!
What were the dimensions of your shed internally again? And is it properly insulated?