The earliest Irish coinage was introduced in the late 10th century,[3] with an £sd system of one pound divided into twenty shillings, each of twelve silver pence.

[citation needed] Parity with sterling was established by King John around 1210, so that Irish silver could move freely into the English economy and help to finance his wars in France.

[4] However, from 1460, Irish coins were minted with a different silver content than those of England,[citation needed] so that the values of the two currencies diverged.

During the Williamite War of 1689–1691, King James II, no longer reigning in England and Scotland, issued an emergency base-metal coinage known as gun money.

[7] The new Free State pound was defined by the 1927 Act to have exactly the same weight and fineness of gold as did the sovereign at the time, having the effect of making the new currency pegged at 1:1 with sterling.

The pound itself was not revalued by this act and therefore banknotes were unaffected, although the 10/– note was replaced by a 50p coin due to spiralling inflation.

[10] The Decimal Currency Act 1970 made additional provisions for the changeover not related with the issue of coins.

It provided changeover information to the public, including a pamphlet called Everyone's Guide to Decimal Currency.

Before this, banknotes were printed by specialist commercial printers in England, and coins were struck by the British Royal Mint.

Despite not being legal tender, British sterling coins of the same shape and size were customarily accepted in Ireland.