A native of Abbey, County Galway, Donnelly was born in the townland of Conicare.
He was educated at the local national school and for one term at St. Molaisses College, Portumna.
He joined the Irish Farmers' Association in 1977, shortly afterwards becoming Branch Chairman.
He campaigned to have all of County Galway classified as severely disadvantaged, successfully improved the finances of poorer farmers.
Since then, Donnelly has continued involvement in farming and promotes quality assured beef from the south-east area of county Galway.