Alas, I'm in the West of Ireland so our locations are a bit far for you. I've a sit on top and I wouldn't do rivers unless they've an obviously slow current or are well used. I find lakes are good, I always go with a paddle partner (I set up a page on face book page with my mates, and anyone I knew with a kayak in my local area to post when I wanted to go out, I always get a response).
So if you're in the West. The Corrib is a great river, full of life, it's wide and after heavy rain has a strong current. Keep to the right of the river (starboard). As for the lake. I've not paddled on the west side of it, only the North east (go it at Lislaughrey for Cong).