The sample illustration above shows the Historic 6" colour map on the left and a matched up modern map on the right. This is a great way of being able to see how places that you are interested in e.g. farms or churches, may still exist in the same place, or may have been swallowed up by urban growth. You can even access great aerial photos to compare to the old maps, which is invaluable in trying to decide where to stick the co-ordinate pin into the cartoon Bing map that is usually the only mapping function most of us have access to in our genealogical software.
I had to ask the OSI how to use the application via the Feedback option - you may be more intuitive than me. I've written up their excellent clear advice on my Irish Maps and Gazetteers page and repeated it on my Townlands page for good measure.
I encourage you to have a play - there are some wonderful historic features you can have highlighted by choosing them from the layers list. So, if gas works, gravel pits, sluices and windmills of the 1830's are your thing, this is how you'll find them!