In The Cookin’ Woman: Irish country recipes (Blackstaff Press, Belfast and Dover New Hampshire, facsimile edition 1986), Florence Irwin says of Lough Neagh:
This lake, 153 square miles, has always provided much sea-food. Eeels [sic], pollan, trout, grunts and their elderly relatives, perch, to mention the most common fish eaten by the loughsiders and sold by them.
No doubt that is true for certain values of “sea”.
She says that the (Irish) pollan is found only in Lough Neagh, but that does not seem to be true. I had not come across the grunt before: earlier, in her chapter on soups, Ms Irwin has a recipe, from Moortown, for grunt soup, which requires 1 dozen grunts; she explains that
Grunts are the young of perch.
Perch we called “grunts” …
which suggests that perch are grunts or vice versa. Wikipedia says
The grunts are a family, Haemulidae, of fishes in the order Perciformes.
The taxonomy set out on its page about Perciformes suggests a relationship between grunts and perch that is more complex than either Ms Irwin or Mr Heaney allows, but says
Classification is controversial.
I am left wondering whether there are still grunts in Lough Neagh and, if so, whether they are of the Haemulidae family or whether the name is simply used locally for perch, young or old.
Here is a page about the Lough Neagh fisheries.