Some basic words:
Siopa = shop
Club = a club
Ospidéal = a hospital
Garáiste = a garage
Scoil = a school
Teilifís = television
Raidió = radio
Feirm = farm
Rige ola = oil rig
1) "i" (= in a) affects the very beginning of the next word. e.g. "garáiste" (=a garage) but "i ngaráiste" (=in a garage).
2) "ar" (= on a) also affects the next word when it's a person, place, thing, dunking a "h" after the first letter! e.g. "feirm" = a farm but "ar fheirm" = on a farm.
You'll be pleased to know that words starting with l, n, r are the untouchables and are never affected!