how to classify this 5 Euro banknotes

https://en.numista.com/catalogue/note201658.html

I have one banknote of this type whose prefix is TC

I have read that T stands for Ireland, but the C what it does mean?
Can I catalogue it as P#20t?

sorry but I don't collect banknotes, sonly coins, and I am inserting banknotes in Numista only for being able to swap them

The second letter is part of the serial number with no further significant information as far as I know.

The second letter isn't important except for allowing more possible serial numbers to be printed. However, prefix VH on Spanish Euro notes mean that they are (very likely) test notes, and carry significant value over its denomination. There are some other prefixes/plates rumoured to be test notes, all from the Europa series, though as of now, not all are super valuable:

€20
Prefix NZ (Austria) - Plate N001
Serial # range Sx*999****** (Italy, no particular prefix) - Plate S004
Prefix VH (Spain) - Plates V003 & V005
Prefix XZ (G&D Munich, Germany) - Plate X001

€50
Prefix VH (Spain) - Plates V012 & V013

€100
Prefix NZ (Austria) - Plate N001 (might not be a test note, value not fully established)

€200
Prefix NZ (Austria) - Plates N001 & N002 (might not be a test note, value not fully established)

For the Italian note listed above, the "x" would represent the second letter, with the asterisks representing numbers (the 2nd, 3rd and 4th numbers in the serial number would have to be 999 for it to be a test note). Since your note isn't a known rare variety, it'd be catalogued as P#20t.

Hope this helps!