Do you consider "crab" and "Crab" to be two different words? The Irish for "crab" is portán. Just as there are grammatical circumstances in English that will cause you to spell "crab" with a capital letter - "Crab", there are grammatical circumstances in Irish that will cause you to spell portán with a séimhiú (lenition - phortán) or with an urú (eclipsis - bportán).
One of the "grammatical circumstances" that cause eclipsis and lenition in Irish is possessive adjectives. This is particularly important for the 3rd person possessive adjective a, which can mean either "his", "her" or "their". To differentiate between a meaning "his", "her" or "their", you use lenition, no change or eclipsis:
mo phortán - "my crab" (singular - lenition)
do phortán - "your crab" (singular - lenition)
a phortán - "his crab" (singular - lenition)
a portán - "her crab" (feminine - no change)
ár bportán - "our crab" (plural - eclipsis)
bhur bportán - "your (more than one person) crab" (plural - eclipsis)
a bportán - "their crab" (plural - eclipsis)
Singular possessive adjective lenite, plural possessive adjective eclipse. The only explicitly feminine possessive adjective a-"her", doesn't cause any change at all.