um isn't really used in some dialects, however.
um is mostly still in use in Munster, but also in the standard. In northern dialects, in part, other prepositions are taken in its place.
Just tráthnóna by itself
Bhí mé ann tráthnóna -> I was there in the afternoon
This is quite beside the point, but the preposition "um" vaguely reminds me with the German "um" ._.
It's not beside the point: apparently the two are related (both eventually from Proto-Indo-European *h₂m̥bʰi ‘round about, around’).
In the English translation. This is an instance where Irish doesn’t take an article, but English does.
When a habitual verb is used, then um would suggest every afternoon/evening.
For the same reason that we say "in the morning" but "at night" in English. It's just customary usage, habits that have grown up over time, and Irish developed different habits than English.
Should this be a valid translation "we eat rice around evening"? If not how would you say that in Irish?
No, it shouldn't - um thráthnóna is a widely used phrase that everyone will interpret as "in the afternoon/evening"
You use thart ar for the around/about sense of approximate times, though tráthnóna is already so non-specific that "around evening" doesn't really make a whole lot of sense, in English or in Irish.
um does mean "about". But just as we don't translate ar maidin as "on morning", um thráthnóna doesn't mean "about evening".
I think it could mean some time around about the afternoon. When I was growing up afternoon was after dinner. Evening was vaguely when it was getting dark but once it was dark it was night, so evening was hardly ever mentioned; maybe it was between getting dark and tea time, but this was only in the winter!
tráthnóna doesn't translate well into English (because "evening" is rather non-specific), but there is no question that the normal phrasing in English for both "afternoon" and "evening" is "in the afternoon" or "in the evening" - you would never say "we played football about afternoon" or "we met our friends "about evening".
I would like more information on this. as an English speaker this is seems more likely to be evening because that is the meal in the latter part of the day. While lunch is midday-ish. But evening can mean the early part of night for me which isn't covered by trathnona. The period between noon and night seems very long, particularly when in summer night may come at a very late time.
The midday meal can also be "dinner" - the prevalence of "lunch" at midday, and "dinner" latter in the day wasn't always the case, and at least in Ireland, the evening meal is still "tea" in many families (teatime doesn't just mean a cup of tea and a biscuit), though dinner is also widely used.
Even the Wikipedia article on "Dinner" says:
Dinner usually refers to the most significant and important meal of the day, which can be the noon or the evening meal.