Hi nomoneynohoney, I'm no expert but here's some thoughts...
I don't think you will make any money on the interest. The interest they give you is too low, and you have to pay 33% DIRT tax on any interest you gain. This is taken at source, so you actually only get 67% of the interest that you would make.
I think the best you can do is to look for a way of not paying bank fees for your deposit/current account, so at least it does not cost you money.
So you need to decide how much you want to keep in a instant access deposit as an emergency fund, and what to do with the rest.
Are you planning on buying a property in the future? Those 15K could be used to start saving for a deposit.
I understand that investing in the stock market only makes sense over a fairly long timeframe, not if you are going to need the money in just a few years. If you are going to use it for a house, it is better to keep it accessible.
To explain the Capital Gains Tax:
When you sell your shares, if you have made a gain (sale price - purchase price): the first €1270 per year is tax free. For anything you gain above that, there CGT rate is 33% tax on it.