I notice there are many gaelics "Scot and Irish". I did only a quick look. If you need something else, look under the first link:
seventeen people = seacht nduine dhéag;
seventeen bottles = seacht mbuidéal déag;
Long time without an answer. It's Welsh for the number 16 : un ar bymtheg - literally one on five-ten 17 would be something like dwy ar bymtheg , though perhaps a native speaker might confirm that.
Welcome to the forums.
As Chaucer already pointed out, there are " many Gaelics". I suspect that Pearl has mixed up the two branches of the "Insular Celtic" language family.
The original question was about the Welsh dwy ar bymtheg (17).
In Irish ithe number would be seacht déag, however I don't believe it's what Pearl was looking for.
I remember explaining this way of counting in Welsh - (I realise it has been simplified since) - to some French people, only to be told quite emphatically that I was counting according to Roman numerals ie II + XV. (In Breton they still have two-twenty, three-twenty and quatre-vingts).
Likewise in the traditional Welsh system, 20 is ugain, 40 is deugain (= dau ugain, two twenties) 60 is trigain (tri ugain) and 80 is pedwar ugain. So 30 is deg ar hugain, (ten on twenty) 50 is deg a deugain, 70 is deg a thrigain (some tricky mutations there!)
The modern trend is to use the decimal forms dau ddeg, tri deg, pedwar deg, pum deg, chwe deg, etc.
I don't know about the Roman idea though. I can see that XVII is two on fifteen / dau (dwy) ar bymtheg, but the Romans counted in tens, not twenties. Latin viginti (20) and Welsh ugain are probably related, but triginta is 30, not trigain, 60.