The piers/breakwaters at the mouth of the Tyne both have rails in them:
But we're never connected to anything else (well, the south one connected to the quarry just down the coast, blocks for the north pier were taken by rail to a jetty on the river and barged across, then by rail down the pier). So far as im aware, rail mounted cranes of a wider gauge and wagons to move stone/concrete blocks for building and repairing it was the only use for them.
I think Tanfield railway has one of the block wagons and the grab, the ruston survives in scotland.
the track on the pier was in use (or at least intact) into the 70s or 80s I think.
This is interesting reading:
https://southshieldslocalhistorygroup.co.uk/index.php/buildings/structures/piers/
And then scroll down for photos of loco workings here:
https://southshieldslocalhistorygroup.co.uk/index.php/transport/railways/
Thry had a ruston 48ds and a peckett similar to the hornby model, there's an interesting prototype for a model!
Of course this isnt an Irish example, but a good account with photos of the building of two breakwaters a mile out into the sea, presumably methods were similar.