The Bible states that when, as a young man, David slew Goliath the giant he took his armor and put them in his tent (1Samuel 17:54). This would include the giant's sword and spear. The first questions we need to answer, however, is how BIG were these implements of war that Goliath used and how big he had to be in order to use them.
The Bible says Goliath's height was 'six cubits and a span" (1Samuel 17:4). Various Bible commentaries place the length of a cubit anywhere from 17 inches (43.2 centimeters) to 21 inches (53.3 centimeters). The length of a span is highly likely around 9 inches (22.9 centimeters). Conservatively, Goliath was 9 feet 3 inches tall or 2.82 meters and likely was taller! Compare this to the average height of a U.S. male which is 5 feet 10 inches (1.8 meters).
Regarding the size and weight of Goliath's armor the Bible says his battle coat weighed 5,000 shekels of bronze. He carried a long spear or javelin when he went to war that was as thick as a weaver's beam (roughly 2 to 2 1/2 inches in diameter). The head of the spear he carried was made of iron and weighed six hundred shekels (1Samuel 17:5 - 7).
It is safe to say that Goliath wore a protective coat that weighed at least 78 U.S. pounds (35.4 kilograms). Several commentaries place the weight of the coat at an amazing 156 U.S. pounds (70.8 kilograms)! The length of his spear may have been as long as 26 feet (7.9 meters). The head alone on it weighed at least 17 pounds U.S. (7.7 kilograms). He was a formidable opponent indeed!
Goliath's armor comes up again later in David's life. While he was fleeing from King Saul, he went to the priests at Nob to get some food. After he ate, he requested a sword. Ahimelech the high priest gave him the ONLY one he had, which is the one used by Goliath (1Samuel 21:8 - 9). But what would a SWORD be doing in the possession of one of God's priests? The Bible says Ahimelech retrieved it, wrapped in cloth, from behind the ephod. It had become a sacred national possession for Israel. We can well guess that the spear also had the same honor.
In the 11th edition of Encyclopedia Britannica (1911) in an article on Ireland, under the subheading of "Legendary Origins, Early History" we find a people called the "Tuatha de Danan" (tribe of Dan) had migrated into Ireland having with them the Stone of Destiny (called the Lia Fail or Jacob's pillar stone).
There were also other things transported to Ireland. The king's daughters (Jeremiah 41:10; 43:6), princesses, and descendants of the house of David also made the journey. It is through them that God would keep his promise to always have one of David's descendants on a throne over Israel. The article also mentions Scythian warriors migrating to Ireland. In Colossians 3:11, we find the Scythians were a major group of people.
In the 1911 Encyclopedia article on Ireland, we find the Tuatha de Danan and Scythian people (many of which were descendants of the Israelites taken into captivity) with genealogical records dating to Noah. They brought with them to Ireland 'the sacred sword and spear.' These weapons of warfare are highly likely the same ones Goliath used to battle King David.