Works on the N61 between Boyle and Tulsk are due to begin this morning and are expected to last well into the summer.
The works were due to begin early last month but were postponed until now.
Roscommon County Council says the road is likely to be closed for the next 16 weeks.
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It's to allow road resurfacing to take place between Shankill and Runaboil.
A map available on the county council website shows the diversion that will be in operation using the R361 from the junction at Frenchpark to the junction at Greatmeadow, Boyle.
