I believe that the first pieces that have dates were issued during Queen Mary's reign (1554 - 1558). Only a few of those coins have dates. There were also some dated coins for Elizabeth I. Prior to then, you the king on the coin will usually give you a time period. Sometimes when the weights of the coins were changed (e.g. "heavy" or "light" coinage) that will give you another clue. Also there is information about when certain mints were in operation and what denominations they made. One of the more confusing periods was during the time of Henry II, and his sons, Richard I and John. All of the pennies (and that was really the only denomination) had Henry's name on them. The way you tell the difference was by the style of the crown and some other markers. It is believed that this is a Henry II penny because the "E" and the "C" are bulky. Note that one old collector put an "H II" counterstamp on this coin. With Richard I pennies, you look for stubble on his chin. This king is better known as "Richard the Lionheart." John (bad old Prince John, later King John) had a different beard style. This is over simplified, but you can see some of what is involved. Why did Richard and John continue to issue coins in their father's name? One dealer told me it was to keep up the confidence in the coinage. Henry II had greatly improved the quality of the British pennies, and they were popular. Perhaps Richard and John did not want to risk changing that. Richard the Lionheart is a big hero in legendary British history, but to me he's overrated with respect to the British. Richard was far more French than he was English. He spent only about 10 months in England, the least of any British monarch. He was far more interested in his French holdings and the Third Crusade than England which he viewed as a boring backwater of a country. John has long had a bad reputation in British history. He was mean spirited and cruel, and wasn't much of a warlord to boot. At one period he had over half money in the country in his treasury from his high taxes. Despite the fact that he hired many mercenaries, the French were on British soil when he died and were close to taking over the country.
Here is the Irish King John penny which I find more appealing. This one has his name on it, and the portrait looks more like a human being. Dealers say this coin is common, and Dan Jones, who wrote a book on the Plantagenet British kings claimed that John had a lot of these coins made to pay his mercenary soldiers. I found it hard to find one that was this nice. There are pieces out there, but many of them have problems. John was not a King of Ireland; he was just a British noble with some clout. Since his father didn't intend to give him any kingdoms when he died, John was known as "John Lackland." Perhaps that helped to put the chip on his shoulder. He had a big one.
Hope you don't mind me jumping in here. The Irish silver pennies are not common finds, I've been detecting for (whisper), over 20years and haven't found a one yet. Really enjoyed reading all this, great pics, I'm starting to think I've labelled a few of my John, Richard pennies incorrectly now!
A quick THANKs to all who responded & help me dissuade some of my ignorance on this fascinating field of numismatics. J.T.