The arms of Ireland. According to tradition, one of the early kings of Ireland was named David, and this king took for arms the harp of Israel's sweet Psalmist. Probably the harp is altogether a blunder, arising from the triangle invented in the reign of John to distinguish his Irish coins from the English. The reason why a triangle was chosen may have been in allusion to St. Patrick's explanation of the Trinity, or more likely to signify that he was king of England, Ireland, and France. Henry VIII. was the first to assume the harp positive as the Irish device, and James I. to place it in the third quarter of the royal achievement of Great Britain.
“Still harping on my daughter.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894