To address your concerns;
He may well be entitled to claim a legal title to the property, and based on the facts you've outlined there's a good chance that he can do so.
If he can there are 2 basic options - administer the estate, or adverse possession. Each has its own benefits and drawbacks. Administering the estate may be faster, but could be more expensive. Either way, tax may need to be paid, and the timescale will be long. Depending on the circumstances, it could take up to a couple of years.
You can of course agree to a sale, but in such a situation it would be prudent to insert a condition to the effect that completion of the sale will happen 7 days after he produces you a folio with him registered as owner. If he's unwilling to do this, just walk away - you're buying yourself into a load of headaches.
You need to consider tax implications - if you agree to buy the property before he has sorted title, you will have to make an application based on adverse possession. Part of that requires production of a certificate of discharge from Capital Acquisitions Tax. You also need to consider your mortgage if you're getting one. Letters of offer have an expiry date, and it's likely that sorting the title will take longer, so a condition that the sale is subject to mortgage approval would be prudent.
Definitely get specific legal advice from a competent solicitor before going any further.