Much of what I’ve learned about old Irish waterways has come from studying the Irish Ordnance Survey maps of the ~1830s and ~1900s (the tilde shows that the dates are approximate: individual sheets were surveyed and published on different dates). However, I had to pay a subscription to get access to them online, so I couldn’t refer visitors of this site to them. Instead, I suggested that visitors consult the free Google Maps and the Griffiths Valuation online maps.
Now, however, the Ordnance Survey maps are available, free of charge, online. “Historic” is the ~1830s maps in colour and “Historic 25i” is the ~1900s maps. Contemporary orthophotographic maps are also available and, best of all, you can overlay a modern map on an old one. Hours of innocent enjoyment and highly recommended.