Contracts for the whole of the civil construction works for the Lee hydroelectric development were in place towards the end of 1952. Work began on the construction of the dam in Inniscarra in February 1953.

Inniscarra dam is divided into nineteen blocks each 45 feet (14 m) long with a water stop between each block. The dam, which is of the buttress type, is 813 feet (247 m) long and its maximum height is 150 feet (45 m) with an 8-foot (2.5 m)-wide walkway along the crest. It is the first and only buttress dam built in Ireland. The buttress-type dam effects a considerable saving in concrete compared with the normal gravity dam. This saving does not in any way take from the safety of the dam. Against this saving, however, must be set off an increase in the area of shuttering during construction and greater complexity of the foundation excavation.

For Inniscarra, it was necessary to remove all unsound rock and the excavation was done in such a way as to cause minimal damage to the foundations being prepared or to adjoining foundations. The usual depth excavated to reach sound rock was 10 feet (3 m). Rock drilling for blasting was done by hand-held Holman rock drills and Polar Ammon gelignite, and a Cordtex fuse was used for blasting. The loose rock was removed by 19 RB shovels, dozer, or by hand as conditions demanded. The total quantity of rock excavated was about 50,000 cubic yards (38,230 cb. m). When final excavation levels had been reached, all loose or shattered rock was removed by barring and picking and the whole surface was cleaned and washed, using a jet of compressed air and water. Pockets in the rock surface in which debris had collected were cleaned by hand.

Excavation on the sides of the valley varied from 0 to 10 feet (3 m) approximately and most was excavated by hand. The workers looked like flies on the valley slopes as they worked. Where possible, though, bulldozers and excavators were used. In the valley floor the general depth of soft excavation varied from 30 to 35 feet (9 to 10.5 m). The material was gravelly and some of it was used for approach road embankments and cofferdams and the remainder was disposed of in a spoil dump. In all, the total quantity of soft excavation on the site was about 150,000 cubic yards (114,682 cb. m). The material was excavated by 19 RB shovels and transported by Aveling Barford dumpers.

The construction of the foundations 50 feet (15 m) below the river level and the diversion of the river during construction presented a major problem on this site. It was very important to get the right quality of rock that could bear the weight of the dam and also withstand the pressure of the huge volume of water. Working 50 feet below the water level could be dangerous as the cofferdams could burst or there could be a seepage of water into the foundation site. The rock had to be constantly monitored to ensure the right quality.

A river diversion channel 1000 feet (305 m) long and 35 feet (10.5 m) wide was cut into rock in the slope of the valley along its southern side. A longitudinal concrete cofferdam was constructed between this channel and the river. This cofferdam was 150 feet (45 m) long, 33 feet (10 m) high and 46 feet (14 m) wide. Temporary earthen cofferdams upstream and downstream of the dam site were erected and the river was diverted through the diversion channel and temporary culverts. The remaining blocks of the dam were built plus the power station. The upstream and downstream cofferdams were removed. The 20-foot (6 m)-wide culvert was plugged with concrete and the river was allowed to flow through the other culverts.