Good plan, Patrick, although it’s hard taking up work already done well! The street version of those shelters were death traps, as the slab roof could collapse if the walls were hit by a blast wave. I have seen many shelters like yours over here and they were often surrounded by an earth bank, overgrown with grass, to soften the blast.
The NI Govt never seriously expected a raid as it was so distant, with relatively small targets compared to GB. As one Westminster official put it, it would be like a shoplifter walking through Harrods to do their pilfering in a corner shop! Sadly, when NIs meagre defences were tested they proved most inadequate.