Question: What replaced the ten shilling coin when the United Kingdom and the Republic of Ireland decimalised their currencies?

Context paragraphs:
British currency in the Middle East: British involvement in the Middle East began with the Aden Settlement in 1839.  The British East India Company established an anti-piracy station in Aden to protect British shipping that was sailing to and from India.  The Trucial States were similarly brought into the British Empire as a base for suppressing sea piracy in the Persian Gulf.  Involvement in the region expanded to Egypt because of the Suez canal, as well as to Bahrain, Qatar, and Muscat.  Kuwait was added in 1899 because of fears about the proposed Berlin-Baghdad Railway.  There was a growing fear in the United Kingdom that Germany was a rising power, and there was concern about the implications of access to the Persian Gulf that would arise from the Berlin-Baghdad Railway.  After the First World War the British influence in the Middle East reached its maximum extent with the inclusion of Palestine, Transjordan and Iraq.  At first, Indian rupees were introduced to Aden and the Gulf States, and later during the First World War to Mesopotamia.  After the First World War, the Indian rupee in British East Africa was replaced by a florin and then a shilling, which eventually replaced the Indian Rupee in Aden by 1951.  Meanwhile, in 1927 a new Palestine pound at par with the pound sterling was introduced in the mandated territories of Palestine and Transjordan to replace the Turkish and Egyptian currencies.  The East African Shilling in Aden was replaced in 1965 with the South Arabian Dinar at par with the pound sterling.

Fifty pence (Irish coin): The fifty pence (50p) (Irish: "caoga pingin" ) coin was a subdivision of the Irish pound.  It was introduced in Ireland on 17 February 1970.  It replaced the ten shilling coin when decimalised, and due to this conversion was introduced a year before Decimal Day in 1971.

Ten shilling coin: The ten shilling (10s) (Irish: "deich scilling" ) coin was a one-off commemorative coin issued in Ireland in 1966 to mark the 50th anniversary of the Easter Rising.  Ten shillings was a subdivision of the pre-decimal Irish pound, worth ⁄ of an Irish pound, making this the highest value coin in the pre-decimal system.

Double sovereign: The Double sovereign is a gold coin of the United Kingdom, with a nominal value of two pound sterling or 50 shillings.  It was first minted using the design by Benedetto Pistrucci in 1820 under the reign of George III and never entered circulation, instead being considered a pattern coin.  As a precursor to the modern decimalised £2 coin it shares a similar diameter of 28.4 mm.

King's shilling: The King's shilling, sometimes called the Queen's shilling when the Sovereign is female, is a historical slang term referring to the earnest payment of one shilling given to recruits to the Armed forces of the United Kingdom in the 18th and 19th centuries, although the practice dates back to the end of the English Civil War.  To "take the King's shilling" was to agree to serve as a sailor or soldier in the Royal Navy or the British Army.  It is closely related to the act of impressment.  The practice officially stopped in 1879, although the term is still used informally.

Ten pence (British coin): The British decimal ten pence (10p) coin – often pronounced "ten pee" – is a unit of currency equalling ten one-hundredths of a pound sterling.  Its obverse has featured the profile of Queen Elizabeth II since the coin's introduction in 1968, replacing the two shilling coin in preparation for decimalisation in 1971.  Four different portraits of the Queen have been used on the coin, with the latest design by Jody Clark being introduced in 2015.  The second and current reverse, featuring a segment of the Royal Shield, was introduced in 2008.

Decimal Day: On 15 February 1971, known as Decimal Day, the United Kingdom and the Republic of Ireland decimalised their currencies.

Florin (British coin): The British florin, or two shilling coin, was issued from 1849 until 1967, with a final issue for collectors dated 1970.  Valued at one tenth of a pound (24 old pence), it was the last coin circulating immediately prior to decimalisation to be demonetised, in 1993, having for a quarter of a century circulated alongside the ten pence piece, identical in specifications and value.

Bank of England £1 note: The Bank of England £1 note was a banknote of the pound sterling.  After the ten shilling note was withdrawn in 1970 it became the smallest denomination note issued by the Bank of England.  The one pound note was issued by the Bank of England for the first time in 1797 and continued to be printed until 1984.  The note was withdrawn in 1988 in favour of the one pound coin.

Five pence (Irish coin): The five pence (5p) (Irish: "cúig phingin" ) coin was a subdivision of the Irish pound.  It was introduced in Ireland on Decimal Day, 15 February 1971 and reused the design on the shilling coin produced for the Irish Free State in 1928.  Some shilling coins remained in circulation until the early 1990s, with the same nominal value as the five pence coin.

Supporting paragraphs: Fifty pence (Irish coin) Decimal Day
Answer: fifty pence