Neither of those phrases/sentence fragments make much sense in English, so it isn't really helpful to try to translate them - you'd have to give a more meaningful context to get any insight into what it is you are trying to illustrate.
To explain that in slightly greater detail, the word "first" is a noun in "the first is there", but an adjective in "the first runner is there". In Irish, céad as "first" is only a numerical adjective (it comes before the noun, most adjectives come after the noun), so you have to add a noun saying the first "thing".
Here are some examples from the NEID of how "first" as a noun in English is translated into irish using céad as a numerical adjective:
"he was the first to qualify" - ba eisean an chéad duine a cháiligh
"that's the first in the series" - sin an chéad cheann sa tsraith
"January the first" - an chéad lá Eanáir
"First" (university degree) - céadonóracha
"First" (Automotive - lowest gear) - an chéad ghiar
Then why do you use the singular indefinite article with it - "a hundred"?
If i was giving you money, I would definitely use the singular - "there's a hundred euro/dollars/pounds".
There are also times when I would say "There are a hundred .....", but "there is a hundred ..." works for me too.