In the second (lower) menu click Files & Download original translation files as ZIP file[1]
Extract the downloaded zip file
Find the translation file (e.g. ga.pot) in extracted /gramps-project/gramps/po/ folder
Open it in (e.g.) poedit & compile it as gramps.mo
Copy this file to /usr/share/locale/<YOUR-LOCALE>/LC_MESSAGES, overwriting the existing file. You will need root access. It wouldn’t hurt to back this up first.
Actually I found a page that let you download just a .mo file. Weblate is a bit confusing because they menus can be a bit different in different contexts. I’d have to go back and figure out exactly what I did, but the steps above do work.
You get the same menu if you click on Gramps in the Project menu and then choose a language (or not).
The only way I can get the menu with Customise download is to click on the Dashboard menu then Gramps/Program — Irish (/projects/gramps-project/gramps/ga/).
Maybe I see a different menu because I started with a search, but it’s a little confusing as these pages look very similar. The breadcrumb for the first is Gramps / Irish, but from the Dashboard you end up on Gramps / Program / Irish.