I've teamed it up with the Abu Sports Mag, but in all honesty, I wish I'd bought another 525 Mag or the 525 Supermag. The Sportsmag does cast well, but the retrieve is slower and there is no ratchet so you can't just leave it in free spool with the ratchet on. You have to adjust the clutch each time instead. As for the retrieve, I've ordered a different gear set for it to give 6.3:1 instead of the 5.3:1 as standard. This just means you bring the hooks in quicker over the rough stuff. If you add on the price of the gears to the price of the reel, I could have bought the Supermag for the same price.
In its defence though, the magnetic control on the Sports Mag is excellent. I'm using it without any centrifugal brake blocks on it and so far, haven't had a birdy.
As a general rough ground/casting reel, my advice would be to go with the 525 either Mag or Supermag. I think if you bought the Mag, you would be wondering if you should have bought the Supermag instead though. You can get them for a good price off e-bay if you are patient and wait for the "right" one to come along.
For most of the Shannon, from what I know of it, you don't have to be into serious distances of over 100yds anyway, so with a good casting technique you'll be in the fishy zone. There are so many venues to choose from that you can pick the ones that suit.
Check out
http://www.shannon-fishery-board.ie/ for more info on the marks.