Más rud é go chuireann tú isteach ar post ,conas a déarfar
" the last date for applications is the 4th of February?
"Is é an dáta deireanach ná an 4ú lá de mo Feabhra?"

Sé an rud is mó atá i gceist agam ná an "ná". Ach.cinnte go bhfui
botúin eile ann. Táim an bhuíoch daoibh.

_________________

{Is geal leis an bhfiach dubh a ghearrcach féin}

Maybe dáta is deireanaí? or as you had it? Or an dáta deiridh?

Is é... ná... is fine

'Sé dáta deiridh le cur isteach a dhéanamh ná an ceathrú lá de mhí na Feabhra.
'Sé an ceathrú lá de mhí na Feabhra an dáta deiridh le cur isteach a dhéanamh.

botún eile: de mo -> de mhí Feabhra

Ná is Munster Irish. But you can use it in Standard Irish as well.
You forgot to insert "for applications" (= le cur isteach). With this part it is okay to use "ná"
Because the sentence should be somewhat complex to use it.

In more simple sentences as e.g. "Is é an data an 4 Feabhra ", ná is not used - even if the new information (the predicate; 4 Feb) is at the end.

https://www.focloir.ie/en/dictionary/ei/application recommends "spriocdháta", deadline, lit. "fixed date":
the application deadline is 2 July = an 2 Iúil an spriocdháta d'iarratais