Yes, tuill is the usual dictionary headword form of dtuilleann. (Since Irish has no infinitives, its verbs have no unconjugated forms; tuill is the second-person singular imperative.) The NEID suggests that tuill is a common translation of “earn”. Cosain has other meanings as well as “earn”.
airgid a dhéanamh is used for "to make money".
to make money hand over fist
cual airgid a dhéanamh
lab airgid a dhéanamh
do shaibhreas a dhéanamh
to make money is easy, to make a lot of money is another thing altogether
tá sé éasca airgead a dhéanamh, rud difriúil ar fad é go leor airgid a dhéanamh
her sole aim was to make money
airgead a dhéanamh an t-aon aidhm a bhí aici
ní raibh de rún aici ach airgead a dhéanamh
There are 3 common uses of "an" - before a noun, it's the singular definite article, before a verb it's the present tense interrogative particle, and when it is used with a hyphen, it's an intensifier ("an-mhór" - "very big", "an-chostasach" - "very expensive". It can also be used with a noun - "an-chóisir" - "a great party", "an-jab" - "nice work, great job, well done!", "an-lá" - "a great day", "an-drochlá" - "a terrible day")