hello there Irish fellow (or lass). Put up a photo of your rockhopper so I can compare it to mine. I think mine is a 98, I bought it used sometime in the early 2000s, cant recall what year. I've ridden mine pretty much consistently as a commuter since then, but I have travelled with it and it handles two rear full panniers very very well, even on twisty downhill mountain paved roads and me pushing it, no frame flex problems.
I have mentioned here before but sometimes circumstances end up that for the ride home I pick up all kinds of things, so if I have one pannier it ends up being very heavy, and therefore lopsided weight wise with only one pannier. The rockhopper has rather thick rear triangle tubes, and even over potholes with a lopsided load, the frame handles this really well.
Ive toured quite a bit and am always impressed with how this mtn bike frame is very solid.
The chainstays if I recall correctly, are about 435mm or 440mm long, this bike has the same medium quality alu rack on it that the previous owner had on it, and it works fine, despite being rather worn in areas from panniers rubbing against it and being used for easily 15 years. I wear size 9 feet, and with my panniers set father back on the rack, I do not have heel strike issues, but it could be an issue depending on your shoe size, or the rack, or the panniers--so no clear answer.
I've used various tires on it over the years, but find that 1.5in roadish tires are a good size, I've used 2in slicks also, but 1.5s roll nicely along on good roads, but are wide enough (38mm) to be good for rough roads, especially if you don't over inflate them for your weight. As you can see in the photo, I still have the original suspension fork on it, which keeps on working and also helps with our rough roads here.