The Irish for "lunchtime" is am lóin, because the 2nd noun in a phrase like that is in the genitive case (the time of lunch). But lóin doesn't happen in isolation, and "lunch" isn't being used as part of a genitive phrase in this case, so lóin is not appropriate in this case.
am lóin - "lunch time"
sos lóin - "lunch break"
bosca lóin - "lunch box"
feoil lóin - "luncheon meat"
Dinnéar is the Irish for "dinner". It's usually the main meal of the day, and, for many people nowadays, in Ireland as well as in much of the western world, that meal has shifted to the evening as 9-5 work patterns keep people away from home during the day.
I grew up eating "dinner" at "teatime", except on Sunday, when we ate dinner at dinnertime.