It sounds like the top of the counter now is planks. You can't tile to them, you need some plywood down first, then a decoupling layer. That layer could be cbu or a membrane like Ditra, EasyMat, one of the Nobel products, or possibly others. Since the ply would need to be screwed to the planks and it would become the new base for the tile, it doesn't matter that the planks are coated with polyurethane.
You'd be much better off finding a mosaic that has the finish you want. Anything you put on top of it will wear off. One of the big reasons to go with tile is it is less maintenance and will look good for a long time. If you want the grout to be shinier, you should consider an epoxy grout.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.