Do miRNA and antisense RNA do essentially the same thing?
Antisense simply means that a sequence is the complement of another. miRNAs are naturally occurring antisense RNAs yes. The "difference" is that antisense RNA is often used for sequences developed in the lab and used for processes such as RNAi.

miRNAs, on the other hand, are encoded by the genome and are used by the cell for regulating gene expression. They do this through the RISC complex, just like artificial antisense RNAs such as siRNAs.

Basically, antisense is the general term, and can be applied to any sequence whose complement exists in the genome. miRNAs are a particular class of antisense RNAs.