Ancestry has a First Edition map of Ireland dating from 1856-1862. It's digitised at a seriously high definition so it's wonderfully clear to view, and as a result, I've lost most of today moving my mouse around the south west Cork area (my father's childhood home) and gawping at some of the land features I've not spotted in previous online and hard copy maps. Ah, my best kind of Friday!
Even if you're not quite the map-head that I have to confess to being, do take a look at this map. It's easy to move around and zoom in to the individual pages of the maps once they've downloaded, and the index to places on the search page is (for Cork and Waterford, at least) excellent, allowing you to land on the correct map page. From there you can zoom in and use your mouse to locate the places of interest at some speed.
(If, instead, you try to boogie with your mouse around the island without using the index, you won't have as much fun. This mode is slow to shift from page to page (from west to east and vice versa) and I never did find the means to 'travel' north to south or south to north.)