Lough Derg is navigable over its total length of just under 40 kms from Portumna at its northern tip to Killaloe and Ballina in the south, and also has access to over 200 kms of navigable inland waterways.

Lough Derg holds huge stocks of fish, being exceedingly rich in trout and coarse species like pike and bream in the lake's depths, tench and rudd in its weedbeds and of course eels, once a delicacy from here sold at London's Billingsgate. The visiting angler, whether in search of outsize specimens or quantities of smaller sized fish, is guaranteed many memorable experiences. Angling on the lake is principally by boat and facilities are available from many boating and fishing centres. Some operators offer complete packages which include boat hire, gillies, accommodation, travel, all meals including packed lunches, bait, tackle and advice on local fishing conditions. The boat user certainly has the advantage, being able to anchor close to deep water or near reeds or bring a dinghy up a small river.

Mythology tells us the sorrowful tail of how Lough Derg got its name. Eochaidh Mac Luachta, King of Mid-Erinn whose fort bordered what is today Clare and Galway, had the misfortune to be visited upon by a wicked bard, Aithirné the Importunate, (sent by King Connor to stir up trouble in a time of recent peace and test what prince would refuse his demands and thereby be drawn into war). Aithirné, whose skill as a satirist was unmatched and who seldom asked for anything easy or honourable to grant, displayed his abilities at the King's court. He then demanded the King's eye for his guerdon (reward or recompense). Rather than be considered ungenerous to a bard and have his people drawn into battle, he tore out his eye there and then and gave it over. The poor King only had one to begin with. A servant brought his blind master to the edge of the Shannon to wash the wound where he informed the unfortunate King: "Alas! dear master, the water is all red with your blood." The King announced then: "Let that circumstance give it a name for all future time...'Loch Derg Dheirc' (Lake of the Red Eye) shall it be called while the Sionan runs to the sea".