Do I have to use "leath" with "i ndiaidh" and "leathuair" with "tar éis"?
There doesn't seem to be. FGB lists both ceathrú chun a sé and ceathrú go dtí a sé.
Hmm I realize now that "leath" probably means "half", and "leathuair" mean "half an hour"! :)
They both mean “after“, but “half past” is used more often with telling time in English than “half after”.
What is the distinction between "i ndiaidh" and "tar éis"? The hover suggestions are the same: past or after.
Yep, I do remember. Telling time was always a mouth full, and a bit of a tongue twister. :)
The cardinal numbers up to 19 are always used with the numeral particle a, unless they are used to denote a specific number of items.
You've probably encountered the exercise for A haon, a dó, a trí, a ceathair, a cúig, a sé, a seacht , a hocht, a naoi agus a deich as well as various other exercises asking you to translate shorter series of numbers.
These "bare" numbers (maoluimhreacha) are also used to tell the time, and they use the numeral particle.
Note that you say a dó a chlog or a ceathair a chlog, whereas you say dhá mhadra or ceithre chat when specifying how many there are of a thing.