Ceacht 25:
cén t-am a tháinig sí?	what time did she come?	[question word] + [n. m1 #pre.#] + [part.] + [v.] +[pron.]	
tháinig sé ar a seacht a chlog	he came at seven o'clock	[v.] + [pron.] + [prep.] +[part.] + [numeral] + [part.] + [n. m1 #len.#]	
a seacht a chlog	seven o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
a hocht a chlog	eight o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
a naoi a chlog	nine o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
a deich a chlog	ten o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
a haon déag a chlog	eleven o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
a dó dhéag a chlog	twelve o'clock	[part.] + [numeral] +  [part.] + [n. m1 #len.#]	
aréir	last night	adv.	
tar éis	after	compound prep.	
ná	nor	conj.	
luath	early, fast	adj.	
ní raibh	(it) was not	[neg. part.] + [v.]	

ar (on): Note that Irish uses the preposition 'on' and in English we use 'at' in the phrase asking about time. Always be aware that Irish prepositions are usually different to the prepositions used in English. Learn the Irish way because if you translate from English, you will usually be picking the wrong preposition to use. Which preposition you use indicates the level of your knowledge of the language.

Tháinig sí ar a hocht a chlog - she came at 8 o'clock

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ar (positive interrogative marker):  This is used with the past tense of all regular verbs and 5 irregular verbs:

Tar - come
Tháinig sé - He came
Ar tháinig sé? - Did he come?
Cén t-am ar tháinig sí? - What time did she come?

If the verb begins with a vowel, the D' is removed from the past tense.

Imigh - go (away)
D'imigh sé - He went (away)
Ar imigh sé? - Did he go (away)?

nor
In negative responses, ná helps provide additional information:
e.g.: An raibh Róisín ann? - Ní raibh, ná Áine. 
=> No, and neither was Ann. (lit.: "not was, nor Ann");