Translation:My brother is big but my father is small.
GnaG Gramadach na Gaeilge - http://nualeargais.ie/gnag/gram.htm
(An English language translation of an Irish grammar written in German!)
NEID - The New English Irish Dictionary - https://www.focloir.ie/
FGB - Foclóir Gaeilge-Béarla - www.teanglann.ie/en/fgb/
(AKA Ó Dónaill, published in 1977, the "standard" Irish-English dictionary)
EID - The English Irish Dictionary - http://www.teanglann.ie/en/eid/
(AKA "de Bhaldraithe", published in 1959, and getting a bit "old-fashioned". Along with the FGB, it was the standard English-Irish dictionary, but the NEID is being developed to reflect current usage in the language)
The teanglann.ie site that hosts the FGB and the EID also hosts An Foclóir Beag, an Irish-Irish dictionary, and grammar tables and audio pronunciation examples. teanglann.ie and focloir .ie are both run by Foras na Gaeilge, and they also fund tearma.ie, a database of technical terms.
I just purchased FOCLÓIR GAEILGE-BÉARLA NIALL Ó DÓNAILL Eagarthóir Comhairleach : Tomás De Bhaldraithe. When I purchased it it was advertised as "the New Irish-English Dictionary". I believe it was published in 2020, but looking at the page where the date of publication is noted, the most recent date I see is 2012
It is technically possible to slow down recordings, such as the ones used on the Irish course, but it is considerably more complex (you have to manipulate the pitch as well as the speed) than feeding a single parameter to a Text to Speech engine that generates the speech, as used in many of the more widely spoken languages on Duolingo.
You can change the speed of the Irish language speech synthesizer at www.abair.ie, though it may not be as useful as you think it will be, because of the "fuzziness" of the voices used there.
You're right about a sheoladh/a sheol, though a phostáil would probably have been a better choice anyway.
But if GnaG implies that uair isn't used for the "occurrence"/"instance" sense of "time", then GnaG is wrong - if anything am is the odd translation, with uair or babhta more common.
From the NEID:
"for the second time" - den dara huair
"I've heard it two or three times" - tá sé cloiste agam dhá uair nó trí
"a few times" - cúpla uair
"the first time" -an chéad uair
"the next time" - an chéad uair eile
"every time" - gach uair, gach aon uair
From the FGB entry for uair:
An chéad uair a bhí mé ann - "the first time I was there"
Den chéad uair - "for the first time"
Níorbh é an chéad uair agat é - "it was not your first time"
An dara huair a thiocfaidh sé anseo - "the next time he comes here"
Ón uair dheireanach a bhí mé ag caint leat - "since the last time I spoke to you" An uair sin - "that time"
Aon uair amháin eile - "one more time"
Gach (aon, uile) uair - "every time"
Uair nó dhó - "once or twice".
Potafocal includes examples like athuair - "a second time", an dara huair - "the second time, Gach re uair - "every second time, trí huaire as a chéile - "three times in a row" and den chéad uair - "for the first time*
GnaG doesn't imply that uair isn't used to mean 'instance' and such, but that it isn't often used in a relative clause, and it instead lists am with the meaning of 'time' there. Here's the exact quote:
The antecedent must be a general term of time / place / reason / and way. A direct relative clause is equally possible here.
e.g.: áit = place, slí = way, caoi = manner, dóigh, fáth = reason, am =time, lá = day, oíche = night, etc. (uair however is not usual)
But, looking at the examples in the FGB (the NEID has problems), it seems that it is used, so you're probably right on that (and GnaG never denies it's used, just says that it is 'rarely' used).
That description is from the explanation of the "adverbial use" of an indirect clause - this sentence doesn't fall into that category, since the relative particle a is not replacing a relative adverb, which GnaG lists as "when, where, who, how, etc. " (an áit a bhfuil mé sásta - "the place where I am satisfied", an fáth a bhfuil mé sásta - "the reason why I am satisfied", an t-am a bhfuil mé sásta - "the time when I'm satisfied", an tslí ar fhoghlaim mé Gaeilge "the way how I learned Irish", etc )
The more I think about it, I wonder whether I should treat it as a conjunction - Sin é an dara huair gur phostáil mé é sin inniu
There are only a few examples like that out there, though:
Sé seo an dara huair go raibh an Tánaiste sáinnithe ina carr le rionnt míonna
Mar a tharlaíonn sé, seo an dara huair go bhfios dúinn gur cuireadh Gaeilge ar an amhrán áirithe seo de chuid Uí Shíoráin
Ba í seo an dara huair gur tháinig Gloria agus Banna an Gharda le chéile d'ócáid mar seo