The Poolbeg Lighthouse is located at the end of the Great South Wall in Dublin where the River Liffey flows into the Irish Sea. The original lighthouse here was built in 1768 however it was redesigned into its present form in 1820. It was painted red to indicate ‘port side’ for ships entering Dublin Bay and the North Bull Lighthouse (on the other side of the bay) is painted green to indicate ‘starboard’. The Great South Wall extends nearly four miles out into Dublin Bay from Ringsend. It was the World’s longest sea-wall at the time of its construction and still remains one of the longest in Europe. Navigation for sea vessels entering Dublin Bay and into the River Liffey has always been difficult and dangerous due to sandbanks and silting. In 1717 a decision was made to built a safe shipping lane with the help of a wall and so the first oak piles were driven in the boulder clay for what was to become the ‘South Bull Wall’. By 1795 the wall was completed with massive granite blocks brought from the quarries on Dalkey Hill. The base was 32 feet thick tapering to 28 feet at the top. The Ordnance Survey Ireland also measured the low water mark of the spring tide on the 8th April 1837 and used this point as the standard height for all its maps until 1958.