There are no islands of any great size in lakes in Wales.
The crannog in Llangorse Lake is an artificial island.
Several reservoirs contain islets e.g. Llyn Brenig, Elan Valley Reservoirs and Llyn Trawsfynydd, the last named having the largest and most numerous, though some are linked by causeways.
Caution is required in that whilst 'ynys' most commonly translates as island, it has other meanings too.
Some of these named places were formerly islands surrounded by marshland.