Lough Sheelin (from Irish: Loch Síodh Linn meaning "lake of the fairy pool") is a limestone freshwater lough (lake) in Ireland located in County Westmeath, County Meath and County Cavan near the village of Finea and the town of Granard County Longford.

Some people would say that Sheelin is the best trout lough in Ireland, some might say it is the best anywhere, the statistics tend to bear this out. In a recent study the lough, which is only 4 miles long and 1 mile wide (4654 acres), was found to hold over 100,000 fish over 8 inches of which 40,000 were between 2 and 4lbs. Now that is phenomenal. The average for the lough is 3lbs with fish up to double figures taken every year.

What makes Sheelin so productive? As with many of the famous loughs, Sheelin lies on limestone giving the water a high Ph. Alkaline water ensures fertility which means everything grows fast, there being so much to eat that the lough sustains a fish population way above what its size would suggest. The lough doesn't just hold trout, there are substantial pike and a large population of coarse fish which becomes important to the trout angler later in the season. Off course there is the fly population which is plentiful making Sheelin a top dry fly venue, then there is the Mayfly which brings the lough to the boil.