Irish Harp
A part of the long history of harps in Ireland, the Irish coat of arms and the Guinness logo are modeled after the same harp at Trinity College.
The harp has been a part of Irish culture & mythology for millennia. The Irish goddess/mortal woman Cana Cludhmor (aka Canola) is said to have fallen asleep to the sound of music by the sea. When she awoke she realized the music was being made by sinews attached to the ribs of a rotting whale. From this she created the first harp (… or possibly her husband did). The Dagda, the chief god of Irish mythology, was said to own a magic oak harp named Uaithne that could fly to him when called. Playing this harp The Dagda could summon the seasons, bring peace, and more. (That said Uaithne may have been the name of his harpist and not the instrument itself.)

An instrument of politics
The coat of arms of Ireland is a gold harp set on a blue background. It’s the only country to have a musical instrument as its national symbol. The harp had been the heraldic symbol of Ireland since at least the 13th century but Henry VIII officially made the harp the symbol of the Kingdom of Ireland when he declared himself king of the land in the 16th century. Also Henry was apparently quite the musician himself, playing the harp among other instruments.
While the harp was an emblematic part of the British crown it was also a symbol of Irish resistance. In the 16th and 17th centuries harp music was enjoyed in the English royal court while simultaneously being outlawed in Ireland. The English government looked to stamp out Irish culture in order to control the people. In 1603 Queen Elizabeth I ordered harpists to be executed and their instruments destroyed. In 1652 the Act for the Settlement of Ireland was intended to subjugate and ethnically cleanse the Irish people which, among other things, again outlawed Irish harp playing.
Irish republicanism and militant groups have long used the harp as a symbol. In 1642 Owen Roe O’Neil used a gold winged harp on a field of green as his flag. The Irish forces in the Rebellion of 1798 used a similar flag. In America the Fighting 69th were a Union regiment in the American Civil War, made up up of mostly Irish immigrants, who had a flag of a sunburst and a harp. During the Irish War of Independence in the early 20th century a green flag with a simpler golden harp, along with the tricolor flag and the plough & stars flag, were used by the Irish fighting against the British.

Brian Boru’s harp / Trinity College harp
The modern harp design on Ireland’s coat of arms, its coins, passports, stamps, etc. is modeled after the 14th or 15th century harp currently on display at Trinity College in Dublin. The Trinity College harp is also known as Brian Boru’s harp, named for the 11th century High King of Ireland, but the harp wasn’t his as it was made hundreds of years after Boru died.
It is unknown who the harp’s original owner was but given the elaborate construction it was most likely a high status individual. Its medieval design is similar to two other harps currently in Scotland and is the oldest Irish harp in existence.

the Guinness logo & the Republic of Ireland
In 1862 Guinness began to use a stylized version of the Trinity College harp in their branding and made it their trademark in 1876. When Ireland became a free state in 1922 the government looked to use a stylized version of the same harp but was faced with an odd problem. While the idea of representing Ireland with a harp had existed for hundreds of years, a stylized version of the Trinity College harp was already a registered trademark of Guinness. The solution: flip it.
The Guinness harp faced to the right and so the Republic of Ireland faced theirs to the left – both are modeled after the Trinity College harp, but face opposite directions.
Added info: created in 1960, Guinness named their lager beer brand Harp and used the same harp in its logo. You can see the Trinity College harp at the far end of the very photogenic Long Room in the Library of Trinity College.
Also the Samuel Beckett Bridge crossing the Liffy in Dublin is designed like a harp laying on its side.