There was no door to the right hand side of the main facade. The reason you see a lintel behind is that it was for supporting something. The black and white photos tells me it is a small roof canopy, my guess being that externally there was some type of big ass crane, the weights etc. being internal. Oh lookeee here.
The roof was originally roofed in Bangor Blue slates, but when a timber roof starts to fail, which it must have, they would have simply taken the slates off, sold them, and re roofed in sheet corrugated metal. No need to fix the sagging roof timbers then, make a few bob on the slates, it probably paid for the sheet material. An accountants type of "building management and conservation"
The canopy is of 1970's or even 80's vintage. That profile of sheet metal for the fascia is relatively modern. They did an identical gunthering at Tuam. But here's the thing, there was a canopy there already. See the structure on the left hand side? Now also look at the stonework, not coursed random rubble - mighty hard to tie anything in later, that's why you can make out the infill where there was an opening onto the platform with the cattle stop. And if I were a betting man, I'd say the door was straight across from the one on the road side, and of identical size.
As for the door openings on the road side trucks would have reversed up to them and parked perpendicular. Now at the turn of the century OS maps there is only a bit of a canopy to that side, which suggests there was only one large door, and possibly a small one. I've seen this before on big ass goods sheds. Clearly they extended the canopy as traffic increased, they opened up the pedestrian door, and then cut another one down the far end. Hence the staggered nature of their spacing. The doors would have been the sliding door type with big lumps of steel to carry the pully mechanism.
There also was a canopy on the platform side too - see the remnants of missing joists and exposed wallplate? How big or otherwise is unknown, but if you check the likes of the Lawrence Collection, it might have a photo of it.
Have a look through here - a mixed bad of UK and Irish wonderment -
And don't forget the depot managers window either, now blocked up. All the infill blockwork would have been done at the same time as the steel canopy was put it.
And the extension at the end of the shed is not original, but built at the same time as the signal box, but you already knew that
The floor would have been a timber suspended floor, removed to take the weight of kegs etc. made up of big lumps of old railway sleepers. Removed in the 70's too I'd imagine.
and
Can't remember what else was asked, but I hope the above is of use. R