While you do want to try your best to use accurate and original names for your characters, be they contemporary or historical, you also don’t want to make the mistake of only using Top 100 names or really out-there names. Does everyone you’ve ever known have a name that fits into one of those styles?
While it’s true that people 50+ years ago were far more conservative and monolithic in naming tastes (just compare the new Social Security list for 2012 to that of, say, 1940), there have always been outliers. Haven’t we all known people with names that were either ahead of or behind the curve? Say, a little girl named Dorothy, a 20-year-old Barbara, a 50-year-old Brandon, or an 80-year-old Jason. Even names that didn’t crack the Top 1000 before they became common were still recorded as being used, like, say, a Bethany born in 1930.
If you’re gut-loading your book with Top 100 names, it’s probably not going to age well. People will know when your book was published if every character has a name like Jason, Jennifer, Jessica, Kevin, Nicole, and Joshua. It’ll look even sillier if you’re gut-loading on the Top 100 names in a historical. Just because there were a few outliers with names like Amanda, Jennifer, Brian, and Liam before they became really popular doesn’t mean they were as common as names like Betty, Mildred, John, George, Mary, and Ruth.
There’s a big difference between a name that was uncommon but still existed, and a name that didn’t exist or just wasn’t used on girls before a certain date. A 50-year-old Isabella or a 30-year-old Liam would be uncommon but plausible. A 20-year-old Kayden or 50-year-old woman named Madison would not exist.
Yes, I have a character born in 1930 named Cinnimin, but that name, either with the “conventional” spelling Cinnamon or my inadvertent spelling variation, has always been very uncommon. I don’t believe it’s ever cracked the Top 1000. But it’s explained that her mother got the final say in naming her, and named her after her favorite spice. The name started out as my own 11-year-old misspelling, but I just like the look of Cinnimin more than Cinnamon. It looks softer, with a more intuitive pronunciation. It’s said that Mrs., later Widow, Filliard isn’t a very good speller. And the name really matches her appearance and personality. There’s a reason for the unusual name.
It’s not plausible to give such a reasoning for more than a few characters with uncommon names. There are only so many times you can get away with claiming that a girl with a name like Jordan or Riley comes from old money and it’s a family name, or that the parents of Aidan or Liam are proud of their Irish heritage. It’s much more realistic to just use names that were common in a given generation.
Re: Irish names, it would be rather uncommon for a historical Irish character to have a distinctively Irish name like Aoife, Saoirse, Declan, or Cian. Sadly, the British domination of Ireland was so overwhelming that most Irish people didn’t openly express their traditional culture for many centuries. If you do your family tree, you’ll probably find that your Irish ancestors had very British names like John, Mary, Elizabeth, and Nicholas. I had a hard time believing the traditional Irish names used in Leon Uris’s Trinity for this very reason.
Sometimes a name did exist, but was only used in a certain language prior to being given a new life with new origins. For example, the Irish name Caitlin is pronounced something like Cot-leen or Coyt-leen. Until about the 1980s, it would’ve been rare to find it on anyone who wasn’t Irish, let alone spelt in the modern 144 verified spelling variations. And Kayla/Kaila was a real Yiddish name long before it became an invented soap opera name in 1982. So if your Caitlin isn’t Irish and your Kayla isn’t Jewish, I’m going to call BS, esp. if most of their friends and relatives also have anachronistic names.
If you’re going to use an outlier name, it’s best to use something from the classical eccentric or classical unusual lists. It’s always best to err on the side of caution instead of look ridiculous by using a name that only recently was invented or became popular. Less people will raise an eyebrow at a historical character named Quintessa or a 20-year-old named Felix than they’ll raise an eyebrow at a 19th century Stephanie or 15-year-old Zayden.