Below I have compiled a list of useful resources that will help any budding military historian, amateur archaeologist or curious cat. One great first step to take is download and install Google Earth Pro (for free) to access some fantastic aerial imagery of the UK and Ireland, and then download and import the Defence of Britain overlay from the Pillbox Study Group. This incredible database will overlay all known (primarily) WW2 sites across the UK, and their last known condition.
Irish Open Street Map Community Map Series (Ireland British War Office, 1:25k GSGS 3906 and British War Office One Inch to the mile, 1941-43 G.S.G.S. 4136)