Definitely 'ar' can be a tricky one. Some small examples:

Tá an fear ar bord = The man is aboard (ship or boat)
Tá an fear ar bhord = The man is on a table

ar muir is ar tír = at sea and on land
ar mhuir is ar thír = on a sea and on/in a country

ar bís = excited / nervous
ar bhís = on a vice

teach ar cíos = a rented house
laghdú ar chíos = a reduction in a rent

... there is lots of examples of this in the language... just to be aware of it...

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.