I wouldn't say it's particularly common or uncommon for people to use their middle names in Ireland - it would be more common in people over 35 I think for reasons I'll explain. There is no set in stone tradition/rule/law(these days at least) that you name your children a saint's name but it was the case until several years ago that a priest could refuse to baptise a child without a saint's name - the last time I heard of this happening was about 17 years ago and it caused quite a stir because it was so unusual.(the name was Sammie-Jo). It's significant that the names you mention are traditional and saint's names and that the female middle names,at least, are names that have only been used here fairly recently(1960s).
Allow me to give a brief history of Irish naming practises as I see them. Until recently(say the 50s) there was a very limited pool of names to draw from: Anne, Mary, Elizabeth, Brigid, Johanna, Ellen, Catherine, Rose, John, James, Michael, Matthew, Thomas, Patrick, William, etc. Irish names like Siobhan and Eamon were very,very rare till the start of the 20th century though names like Liam and Seamus would have been used as nicknames. Really it was unusual for families to deviate from a defined pool of family names. My grandfather was John, his father was Thomas, his grandfather James and except for his great-grandfather all his direct male descendents were called John right back to the late 16th century.
You would name your first son after the father's father, the first daughter after the father's mother, the next two after the mother's parents, the nest two after yourself and then after that a favourite sibling/one who died/became a priest or a nun/emigrated. My father's family is a perfect example of this - three daughters - one named for the two grannies and the mother, 6 brothers named for the two grandfathers and the father and for two uncles. My grandmother then "ran out of ideas" and called her last son after the saint's day he was born on(quite a common practice).
Because you could have 3 or 4 people in one house with the same name, nicknames,based on the given name or more rarely a physical attribute, were common as were the use of compound names, like Mary Teresa, Mary Anne, etc.
I think it became more common in the cities and towns for people to start using second names - just because of the numbers of people. Also city/town dwellers started using names that were not family names/mixing family names with non-family names. There was still a heavy bias towards saint's names though - but you can see a pattern of people going towards more exotic religious saint's names as the twentieth century progressed - e.g. the rise in popularity of both Gerard and Majella; Pius and Marian; Concepta and Carmel - religious names but very twenthieth century.
In the 1960s Ireland opened up in a dramatic way because of increased prosperity, better communications and emigration. Parents began to realise they had more choice in picking their children's names. There was a sharp increase in the use of names like Lorraine, Denise, Darren, etc. However the old tradition of using family names(especially grandparent's names) and also the tradition of giving a saint's name was strong, for a while anyway.So you would have girls called Mary Yvonne who went by Yvonne. This tradition would have lingered on in country areas longer. In school in the 1980s I knew an Elizabeth(family name) who went by Lisa and a Edith Rachel who went by Rachel - this was rare though.
** The short answer about is that a) until recently there was a strong tradition of naming children after parents and grandparents and b) for years priests wouldn't christen children unless they had at least one saint's name. But in the 1960s parents began adding "more exotic" names and using them instead. The last practise died out years ago except for rare cases. I bet if you asked the people you mention they are called after a family member. I would be interesed in hearing where they are from and what age they are.