okay...I hate asking this because I hate when people try to shoehorn stuff like this for names of bands or pets etc...however, I'd like to know my options with this...

I have written a series of short stories for children (in english) titled The Adventures of Finbar Greenjacket: or, ten thousand years in the life of a leprechaun.

Finbar Greenjacket is the title character, and I would like the next series of stories (or possibly a web-based comic strip) to be as Gaeilge, but I don't know what to do with the name. So far, my leading contender is Fionbar Casóglas (or need the 'g' be doubled there?), but I would like to know if those of you who are more experienced than I would prefer Glascasóg or if there is something that might make sense and be a phonetic (but not translation) near-equivalent of "greenjacket"

thoughts?

thanks in advance

Finbar Greenjacket

Finbar na Casóige Glaise

Finbar Ó Casóguaineáin - A fine fictional name. :-)

what is the difference between uaine and glas? and would Finbar Ó Casóglasáin be as acceptable as Casóguaineáin? Also, why the 'áin'? Would Casóguaine work too? This sort of 'invention' is new territory for me...

Uaine is always the colo(u)r green. Glas can be green or grey or blue depending on context.

"áin" is a suffix which might mean "ite" or "ist" or "small one"

I understood 'áin' to be a diminutive suffix on some Irish surnames. I came up with that name as it is similar in construction to my own. One which probably began as a nickname or epithet attached to a proper name where over time the proper part was forgotten and dropped.
If Finbar is the first and original Greenjacket, (I'll assume it's what he wears and everyone knows him for it.) then I'd go with the example that Fear na mBróg gave above. If he comes from a lineage of those who came from an earlier ancestor - though he himself also now wears a green jacket any way, you might consider the example I gave as an option.
And , just a thought, you could play with the name in numerous ways to create other characters who could be related to and/or descended from Finbar. MacCasóguaine, MacCasóguaineáin, Ó Finbaruaine, etc...

The suffix really is -án... while -áin is the genitive form of it.

Could it be used as a genetive modifier. I'm not sure how exactly to ask that question really. Mar sampla, Casóguaine - Greenjacket, MacCasóguaine - son of Greenjacket, Also Casóguaineán - son of Green jacket (Little Greenjacket), Casóguaineáin - related to/descended from Greenjacket??, or to Little Greenjacket??, Ó Casóguaine - grandson of Greenjacket, Ó Casóguaineán - gr.son of Little Greenjacket??, Ó Casóguaineáin - descendant of Greenjacket??
Of course, after so much time is passed, names like Finbaruaine, Ó Finbar, and even Finbaráin could be presumeably found.
Is there any sense to this?