Bus Eireann is asking Kerry passengers to use buses at off-peak hours if possible.
The company says some Expressway intercity coach services in the county are experiencing demand over the reduced capacity allowed under COVID-19 regulations.
Bus capacity is strictly limited to 50% to adhere to public health guidelines.
More than 10% of services on Route 40 Tralee/Cork/Waterford/Rosslare, serving Tralee, Farranfore, Killarney and Glenflesk have reported as full since the end of June.
The busiest times are from 11.00 am to 1.00 pm and from 3.00 pm to 7.00 pm; there’s more demand at weekends, especially Friday afternoons and Sundays.
Bus Eireann says it provides live updates on its services on its Twitter feed (@buseireann) to help passengers plan journeys.