The value of old Irish banknotes could have at least doubled over the past three years.

According to Mr Ian Whyte, director of Whyte's auctioneers in Marlborough Street, Dublin, the values of early 19th century Irish banknotes can range from around £50 up to £500-plus, with most of them fetching £100 to £300.

Later 19th century banknotes, which generally had a plain design using a figure like Hibernia with the harp, tend to fetch between £50 and £300. But "you don't see that many of them around", he says.

Early 20th century Irish banknotes are reasonably scarce, "maybe even scarcer than the 19th century notes are. Possibly there's a bit more demand for them because they're more attractive looking. They tend to fetch, in good condition, £150 up to £500," Mr Whyte says.

Until 1928, all the banks issued their own notes. Then in 1928 the Currency Commission took over. It decided to issue a one-design banknote with a ploughman on it. There were £1, £5, £10, £20, £50 and £100 ploughman notes but the £20, £50 and £100, according to Mr Whyte, "don't seem to exist anymore". He adds: "They were all cashed in. There is a rumour that there is a £20 out there somewhere but I have not seen it."

The names of the banks - such as Provincial Bank, Hibernian Bank, the Munster and Leinster Bank or the Royal Bank - continued to be printed on the banknotes. Particularly scarce are banknotes from the smaller banks like the Royal Bank.

The ploughman £1 notes from the period 1928 to 1939 can be worth from £20 to £100-plus, depending on condition. A £5 ploughman note should fetch £50 to £200, subject to condition, while a £10 note should fetch "£150 upwards".

The £20, £50 and £100 only exist with "specimen" overprinted - the kind that were sent to banks internationally as samples of Irish notes. Mr Whyte says a few of these sets have come on the market mainly from places like the US where banks have closed and sold off their archive. "We've sold a complete set of the ploughman specimen notes from £1 up to £100 for £6,000," he adds.

When it issued the ploughman notes, the Currency Commission simultaneously issued its own banknotes with the Lady Lavery design. A specimen set of these "only" fetched £2,500 because the notes exist without the specimen overprint. "Generally speaking, collectors prefer the notes without the specimen overprint. But if they can't get anything else, they'll go for them," he says.

If you have a stash of £1 Lady Lavery notes from the 1970s, the bad news is they don't fetch a lot of money. "They might fetch £2 each or something like that. But ones from the 1940s and 1950s can be worth £5 to £10."

Printing errors on banknotes "don't fetch the same prices as similar errors on postage stamps. Our recent errors would have been the 1978 £20 note - the W. B. Yeats one". One of the colours on the reverse of the note was omitted resulting in no inscription: it didn't say legal tender or Central Bank of Ireland. "They've been fetching around £200," says Mr Whyte.

While selling old banknotes could realise hundreds of pounds, keeping new ones would not seem the wisest investment. For example, a £100 banknote from 1937 in reasonable condition might fetch £200 or £250. But "when you consider what £100 would have bought you in 1937, it has devalued. If you got one in uncirculated condition, you'd probably pay about £500 for it. But again it's still not reflecting the value it had", he says.

Whyte's will be publishing a book on Irish banknotes next February.