The getNodesTaggedBy method might not always return the correct results.
On line 103, the clauses String is joined with the queryString, there's no separation of the different clauses though.
When using the logical operand "OR" you might get this query;
SELECT * FROM [nt:base] WHERE ISDESCENDANTNODE(/myFolder) AND [mgnl:tags] = 'myTag' OR [mgnl:tags] = 'mySecondTag'.
The "OR" in this clause negates the first ISDESCENDANTNODE condition and will give all content tagged with 'myTag' or 'mySecondTag' and not only under the selected folder.

The clauses variable should be encapsulated between parentheses "(" and ")"