On my page about the Broadstone Line of the Royal Canal, I say this:
In 1845, the Midland Great Western Railway Company (MGWR) bought the Royal Canal. Doing so allowed it to run its lines beside the canal, which it did most of the way to Mullingar, without having to conduct lengthy negotiations about wayleaves with individual landowners. The company built a terminus at the Broadstone, with a pontoon bridge (which was moved out of the way when boats entered or left the harbour) to provide passengers with access to the station.
I have found a picture of the arrangement. Here it is.
The pontoon looks rather solid, but there is a canal-boat heading for it and the canal clearly continues on the far side.
I have added the pic to the Broadstone page.