Ceacht 14:
sí	she, it	pron	
tá sí	she, it is	[v] + [pron]	
níl sí	she, it is not	[v] + [pron]	
an bhfuil sí?	is she, it	[interr. vb. part]+[pres. dep. form v. # ecl.# "bí"]+[pron]	
nach bhfuil sí?	isn't she, it	[neg. interr. vb. part]+[pres. dep. form v. #ecl.# "bí"]+[pron]	
ag teacht	coming	“ag” + [vn]	
ag damhsa	dancing	“ag” + [vn]	
ag éisteacht	listening	“ag” + [vn]	
ag caint	talking	“ag” + [vn]	
ag obair	working	“ag” + [vn]	
ag foghlaim	learning	“ag” + [vn]	
cén fáth?	why?	"? phrase":[int. pron merged with art]+[n f3]	
fós	yet	adv	
ag foghlaim damhsa	learning to dance	“ag” + [vn] + [v.]	
ceart	right	n m1	
ceart go leor	right enough	[n m1] + [adv]	

The verbal noun:  This use of the verbal noun is equivalent to the English present participle - the "-ing" part of a verb, as in "coming", so it is used to describe an action in progress.  When used in this way it is usually directly preceded by the preposition ag (at).  It may help to think of it in terms of the way some people still use the English 
"She's at coming" or, more frequently, "She's a' coming", often found in some  poetry or songs.

The Irish verb for "come" is tar, another one of the 11 irregular verbs, and the verbal noun is teacht, so we find:
Tá sí ag teach - She is coming.
Níl sé ag teacht - He is not coming.
An bhfuil tú ag teacht? - Are you coming?
Nach bhfuil Nóra ag teacht? - Isn't Nora coming?