It would be rare, but not very rare. Lots of people would still have them.
The Eireann 1931 "Proof" Shilling is quite rare.
If you refer to the 1920 British Shilling, it is not rare.
500 euro
value of irish penny 1931
The Irish shilling (and pound, shilling, pence system) existed in various forms from 997, but the modern Irish pound didn't exist until 1928 and became a decimal currency in 1971 like the UK that did not use the shilling as a unit of account (however, for a time the shilling coin circulated along with the 5(new) pence coin)
The Irish Press was created in 1931.
Most British One Shilling coins from the past 100 years would not be considered rare. For more detail, you would need to provide the date on the coin.
There were no Eire (Irish) 1957 Shillings minted.
A black hog is a historical term for an Irish shilling, of 13 pence value, as opposed to the white hog English shilling of 12 pence.
Rare Earths Facility was created in 1931.
Frank Fay - Irish actor - died in 1931.
Gerard Lynch - Irish politician - was born in 1931.
1931 is not a rare date for these ... in average condition, it's worth about a dollar