Have you made a gain on the shares?
Are they Irish listed shares?
You're making a gift, at market value of the shares, to your dad.
It's a capital gains tax trigger for you. You'll pay tax on any gain. But if you've made losses, you can't use them as it's not an arm's length transfer.
He pays stamp duty on receipt.
You'll both pay broker commissions.
Why transfer shares?
Is it no cleaner to sell the shares, give cash to your dad, and let him do what he wants with it?