It's what you think you know that kills you.

 So today I  am on Twitter and I read a post about slavery in early medieval Ireland.

 https://twitter.com/thecelticist/status/1426591761539997699

Yes, slavery is a horror. Let there be no argument about this. 

 I note that Irish and British early medieval historians downplay the importance of slavery in both of their societies  Dublin was the slave market for a lot of the early medieval period, and 10 percent of the population of England were slaves at the conquest. 

So a cumal is a unit of value- a slave woman - an ounce 25 grams of silver or three milk cows.

I had it in mind that the word Cumal is from a Latin loan word for girl. Loan words from Latin to Irish are not unusual. Long for Ship, is believed to come from Latin. Although if I recall correctly Latin and Irish show a thousand years of linguistic drift So about the same as English and Danish 

The theory  I believed was that Cumal had come to the Irish language via the slave trade with Roman and later sub roman Britain a coin for a slave girl  

Turns out I was wrong. Firstly there seems to be no link between the word Cumal and Latin. So no link to the slave trade. No link to the word coin

Well, you are always going to be wrong. But what interests me is where did I get the idea from? I was sure it was a Latin loan word. It does not seem to be at all. The Icelanders used the same idea in their culture

Just shows how unreliable our memories are 

Update:  It's in early medieval Ireland.