It is a bit odd to be given a 10% shareholding in a company straight up. It would be more usual to be given an option to buy the 10% if you meet certain targets.
Are there some conditions attached? Do you have to stay in the company for a number of years?
Shares are only worth something if you can sell them when you want to. So you would need to have a valuation in place. That the other owners would have an obligation to buy them at say 6 times pre-tax profits. Many people have minority interests which are worth nothing because no dividends are being paid.
I would suggest that you treat the shareholding as a bonus. You should first be satisfied that you are getting a good job and a good package. If they are trying to give you a shareholding in the company so that they can pay you well below the market, I would be very suspicious of it.
Decide on the business considerations first. If they fall into place, then look at the tax aspects. It is a complex area and I am not a tax advisor. My understanding is that if you get shares, they will be given a notional value and you will be subject to income tax. The last thing you want to do is to pay tax on something, which you can't realise any value for. The firm's accountant would have to do a calculation for the Revenue which would probably be based on around 6 times earnings. This is a very, very rough guide.
Brendan