"This is our profession."
Translation:Is é seo ár bproifisiún.
You can, and the é is also not mandatory and in particular, it would be omitted in Ulster Irish. I haven't read about it in detail yet (see here in the German version of GnaG. The old English version doesn't have that), but apparently the meaning isn't exactly the same… the short form Seo (é) ár bproifisiún. has a stronger indicative nature (for example for introducing something or someone, I think), whereas the long form, Is é seo ár bproifisiún. would rather be used to determine/differentiate something or someone (the example that is given there is Is é seo an múinteoir. = This (this one and not that one) is the teacher.). But Duo doesn't accept them :(
This type of copular statement, with an demonstrative third-person pronoun as the complement, doesn’t need any subcomplement. If the sentence were instead “It is our profession”, where the third-person pronoun was not demonstrative, then “our profession” would be the complement and a subcomplement would be needed: Is é ár bproifisiún é.
I'd say that sounds more natural to me too (ulster Irish, not a native speaker, but fairly fluent years ago). Can't back it up with academic terminology or anything, it just feels "righter"! Though I'd have used the lenited "phroifisiún" rather than the eclipsed "bproifisiún"
Still marked wrong though!
There is a difference between a classification statement ("X is a Y") and an identification statement ("X is the Y"). A possessive adjective like mo or ár acts like a definite article (you are identifying a specific Y), so this exercise is an identification statement, whereas is bean í is a classification statement.