Here “i r” uses the “r” letter “sound”.
first
dirt
firm
shirt
circus
skirt
circle
stir
bird
birthday
circulate
whirl
aspirin
birch
birth
blackbird
circumvent
confirm
dirty
flirt
girl
Kirk
mirth
quirk
shirk
sir
squirm
squirt
swirl
third
thirst
thirteen
thirty
twirl
virgin
15
16
