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.
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.