Your family members are wrong. Even if a printed book of title exists and someone has a copy, mere possession of the book doesn't give him any basis to claim that he owns the property.
The point of the book of title was to show that the person who is selling the land has the right to sell it to you. Large parts of what is now suburban Dublin (and other cities) were originally estates that were farmed. Over time, as the city expanded, they were subdivided, developed and sold off for housing. A single farming property might be subdivided into hundreds of smaller house sites.
Normally, when you sold land, you handed on the title deeds to the purchaser. But, obviously, when you subdivide a large property into hundreds of small properties and sell them off, you can't do that. So what the estate-owners would do is print off copies of the title documents, and give a set of printed copies (the "book of title") to each purchaser, together with a certificate to say that these were accurate copies of the title deeds held by the vendor. As the property is sold and resold, from owner to owner, the book of title gets passed on.
Right. The need for much of this passes when property title gets registered in the Land Registry. The old books of title still exist and still mostly get passed on, but they no longer have the same importance. If Bill is selling you his house, you no longer need to verify that he bought it from Jack, who bought it from Mary, who bought it from Joe, who bought it from the Pembroke Estate Company, who had the right to sell it because, look, here's a book with all their title deeds in it. If the property is registered to Bill at the Land Registry, then you know Bill is the owner and can sell it to you.
So, at some point the booklet of title for this house has been lost, or mislaid, or thrown away - because it's no longer needed. If your solicitor is happy that you can buy the house, then he's probably right. If somebody else has the book of title and produces it, it doesn't prove that they own the property, or have any claim to it. All it proves is that back in 1912 (or whenever) the Pembroke Estate Company (or whoever) owned the property and had the right to sell it to whoever bought it in 1912.