I can’t put extracts from Ordnance Survey maps on these pages, so I’ll have to ask you to go to the OSI pages, do some stuff and come back here.
Go to this location on the Fergus estuary. You can zoom out (minus) to see where you are, then zoom back in. Click on Historic 25″ to see what it was like around 1900 and then click on Historic 6″ (or Historic 6″ B&W) to go back to the 1840s or thereabouts.
Unidentified building at Islandavanna
The oldest map shows Islandavanna as a real island but it is no longer an island: the land around it has been reclaimed. The straight line and the grid pattern of the drains are give-aways, but even the oldest map shows an embankment at Drumquin Point (it’s easier to read the legend on the B&W version).
The high ground of the original Islandavanna
The embankment looking upstream
There is an enormous amount of reclaimed land on the estuaries of the Shannon and the Fergus. You can spot some of it on newer maps by looking for straight lines along the shore and rectangular drains; you can examine it in more detail on the OSI maps, noting what work was done in the latter part of the nineteenth century and what was done since then. Most of the rivers in the upper part of the Shannon estuary have been embanked and there are sluices everywhere.
The sluice and embankment at Islandavanna. Note the car on the right-hand side
James Lyttleton, Aidan O’Sullivan, Andrew J Wheeler and Michael G Healy (“Coastal landscapes and environmental change in the Shannon estuary” in Aidan O’Sullivan et al Foragers, farmers and fishers in a coastal landscape: An intertidal archaeological survey of the Shannon estuary Royal Irish Academy, Dublin 2001) say that reclamation may have begun in the late medieval period. They say there are about 5900 hectares of reclaimed estuarial alluvium in Co Clare (Shannon and Fergus estuaries) and 5601 hectares in Co Limerick (Shannon, Maigue and Deel estuaries). Just for comparison, Wikipedia puts the size of Limerick City at 5130 hectares.
These corcasses (the word is still to be found on OSI maps) are
… below spring tide levels and [have] been reclaimed using a system of dykes, sluices and sea walls.
Outside the embankment, looking across to the Fergus, which flows from left to right
Under the Arterial Drainage Act 1945 (Lyttleton et al, op cit), the responsibility for the care of the embankments was transferred to the Office of Public Works (OPW).
The sluice gear 1
OPW created more embankments, notably after Coonagh (near Limerick) was flooded in a storm in 1961. The embankments don’t just protect agricultural land: Shannon Airport is on low-lying land, as are some houses and places of employment.
The channel leading to the sluice
I was told locally that OPW had recently carried out work at the Islandavanna sluice.
The sluice gear 2
However, I later met some farmers who told me that OPW was hoping to transfer responsibility for maintenance of the embankments to the local farmers.
The sluice gear 3
They said that OPW had funding to carry out some major works: the excavator, barely visible on the skyline in the next photo, was said to be working near Ballycorick Creek.
The (barely visible) excavator
The farmers said that high tides were getting higher, flooding places that had not been flooded in recent times, but that farmers could not afford to raise the heights of the embankments.
Inside looking out
Outside looking in
The farmers also said that there were environmental restrictions on what they could do to the embankments and how they could do it, all adding to the cost.
I see that the matter has been raised in the Dáil.
It seems unlikely that there will be much public money available to improve the embankments.
Irish waterways history 
Thanks for the info., very interesting. Surprising that we did not develop an active windmill powered system of land drainage in Ireland……….but perhaps the ratio of land involved is too small. The area is known locally as ‘the sloblands’. Also, there seems to be no evidence of sowing potatoes in bawn (English ‘lazybed’) on the mud as can be seen on the shores of Lake Titicaca, Perhaps there was a fear of ‘improvement’ and higher rent.
I suspect that pumping (by windmill or otherwise) would not be needed if the drains could be given enough of a fall to empty themselves at low tide. This article on polders may be of interest. Maybe Drinagh, in Wexford, was too low to drain itself.
On slobs, I came across a brief mention today of the difficulty that the Board of Works had, in the 1850s, in finding anyone to take the Clonakilty slobs off its hands after they had been drained.
bjg