What county GAA ground was the last to be built on a new site?

Do you mean Ardfinnan Tipperary ?

Salthill was built in the 1950s which is the newest I can think of off the top of my head.

Be a fair stretch to even call that "borderline"

It's a great question OP.

I can't think of any in my lifetime which is 50 odd years.

Back during the Celtic tiger there was big talk of Cusack Park Ennis being sold and the GAA moving to a green field site.

Same with Stacks Park in Tralee.

Never came to fruition.

Celtic Park is still their main ground, but Derry make so much use of Owenbeg I'd call it their co-county ground at this stage. They haven't played a game in Derry since 2020.

Not sure when Owenbeg was opened but it was in the last decade.

Fairly certain there was talk around the same time in Mullingar around relocating out of Cusack Park for a similar deal, once again didn't happen

I think the last main county ground to be built on a new site is Dr. Hyde Park in Roscommon, which was built in 1969.

They used to play in St. Comans Park, on the far side of the town.

Healy Park in Omagh only had construction started on it in 1968 and formally opened in 1972. Wasn't named Healy Park either until 1980.

Still a good age compared to the Owenbeg facility in Co. Derry which is a spring chicken in comparison though not strictly speaking a county ground AFAIK.