It all depends which fen your on. I've been in Ireland over 20 years now but I used to do some rabbiting on the fens a lot of years ago with various people, some of them on here.
In case you've never been to the fens, it's low lying land drained by canal-like drains of various widths and depths. Many of these have raised earth banks on both sides and that's where hares can escape into cover and where pockets of rabbits sometimes get well dug in. Obviously you can ferret or lamp but the banks are good for a daytime mooch too. You need to take a stout stick and lightly fill in the holes as you