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No real knowledge here, just an educated guess:
I'd assume Liemertje might be a (West) Frisian name, a feminine form of Lieme (via the diminutive suffix -tje): https://www.meertens.knaw.nl/nvb/naam/is/Lieme
Lieme itself is apparently a variant of Limme: https://www.meertens.knaw.nl/nvb/populariteit/naam/Limme
In that case, the first part of the name would be derived from the Germanic name element "liud-" = "people" and the second element would be derived from any of a number of Germanic name elements beginning with "-m-", such as "-mar" = "famous".
See here: https://www.meertens.knaw.nl/nvb/verklaring/naam/Limme