It's not only Irish.
I remember as a child in Canada in the 1950s and 1960s that it was quite common.
We lived in an apartment, but when I visited friends' houses the front room was off limits to us. This room was always kept clean and tidy and had all the expensive ornaments, books, decorations, etc. Typically the furniture was covered in plastic to keep it clean for when special guests arrived.
Except at formal occasions, no one used the front door.
Even adult friends and neighbours would knock on the back door.
The front door and front room were for formal use only.
Compare with a 1960 Brooklyn housewife' dreams of suburbia (Somewhere That's Green):
I cook like Betty Crocker,
And I look like Donna Reed.
There's plastic on the furniture,
To keep it neat and clean,
In the Pine-Sol scented air,
Somewhere that's green.