Airports:
Missoula, MT (MSO): Lake Mary Ronan is less than 2 hours drive from Missoula International airport. (Easiest option from Europe)
Kalispell, MT (GPI) Glacier National Park Airport. 1.5 hours drive to Lake Mary Ronan
It you are planning a roadtrip through the west of the USA, with a stopover in Montana, there are direct flights to:
Denver, CO: 1500km south east of Lake Mary Ronan -- Yellowstone Park is “on the way” to Montana as well as Teton National Park.
Salt Lake City, UT: 1000km south of Lake Mary Ronan
Seattle, WA: 800km west of Lake Mary Ronan
Another alternative is to fly first to Canada and the road trip through Glacier National Park to Lake Mary Ronan
Calgary, Canada: 560km north of Lake Mary Ronan -- 6 hours drive through beautiful scenery.
By Car:
From Missoula: Follow the Highway 93, about 20min after Polson (last supermarket) turn left towards Proctor. Follow the road towards Lake Mary Ronan and turn left onto Lake Mary Ronan Drive.
Google Link
By public transport:
Not an option...sorry...but feel free to cycle...