。
To me, she's irreplaceable.
Literal
I [for-にとっては] irreplaceable person [explanatory-なんだ].
かけがえのない ('irreplaceable') is a fixed adjectival phrase — literally 'having no substitute.' かけがえ is 'substitute, replacement,' の+ない marks 'no.' The expression is solemn and emotional, typical for describing beloved people or things. にとっては adds the contrastive 'to me specifically.'