Ceacht 24:
níor	negates a verb	part.	
níor imigh sé	he didn't go off	[part.]+[v.]+ [pron.]	
níor tháinig sí	she didn't come	[part.]+[v.]+ [pron.]	
níor rith sé	he didn't run	[part.]+[v.]+ [pron.]	
níor shiúil sí	she didn't walk	[part.]+[v.]+ [pron.]	
níor thit sé	he didn't fell	[part.]+[v.]+ [pron.]	
níor léim sí	she didn't jump	[part.]+[v.]+ [pron.]	
anocht	tonight	adv.	
tinn	ill, sick	adj.	
go luath	early	[ part.] + [ adj.]	
d'imigh sé leis	he went off (by himself)	[v.]+[pron]+[prep.pron]	
tuirseach	tired	adj.	
a chodladh	to bed, to sleep	[prep.] + [ vn.] 	

níor:  This negates a regular verb, and 5 of the irregular verbs, in the past tense. 
Tháinig (past tense of tar - come) is one of those irregular verbs which use this negative marker.

See the notes for Lesson 23 on forming the past tense.  
Imigh (go [away] ) is a regular verb.  If the verb begins with a vowel, the D'  is removed.

D'imigh sé - He went away.
Níor imigh sé - He didn't go away.