I knew Liam fairly well and even did a little business with him, selling his surplus stock of CDs back in the 1990s. He was a very fine "ballad" singer (Jim Carroll correctly has it that "ballad sessions" in Irish Pubs were not necessarily confined to that type of song). He took the Ireland of the 1960s by storm, along with his brothers and Tommy Makem, with their energetic brand of performance. It had to be experienced live, I have always believed, to really get the very best from it.
His brother Tom had a really fine voice also, possibly with a greater vocal range ?.
Their acting experience (They were regular "off-broadway" thespians in the 1950 -60s before the "Folk Revival" took hold in New York) must have been of great benefit when they switched to being mainly singers.