Numbers 1-6 dunk a "h" after the first letter - in general!
Numbers 7-10 affect the very start of the first letter - in general! (See "Before You Start : Pronunciation and Stuff : An Béal Bocht" section in the Little Black Book over on the left)
But rules are made to be broken, as Colin knows by now so don't freak out when you see these few exceptions :
"pingin" (penny),
"bliain" (year),
"ceann" (head),
"uair" (hour)
Irritatingly, they change their form when being counted from 3-10......just trying to show off!
3-6 pingine / bliana / cinn / huaire
7-10 bpingine / mbliana / gcinn / n-uaire
But don't worry, these are the exceptions!
40 = "ceathracha" but some people like to use "daichead" for 40. Colin prefers "ceathracha" because it reminds him of the Irish for 4.
41 Daichead a haon
42 Daichead a dó
43 Daichead a trí
44 Daichead a ceathair
45 Daichead a cúig
46 Daichead a sé
47 Daichead a seacht
48 Daichead a hocht
49 Daichead a naoi