Complex sentence: Joseph MacManus' father, Seán MacManus, who at the time was a leading republican went on to become Mayor of Sligo, the first Sinn Fein Mayor in the Republic of Ireland since the beginning of the conflict in 1969.
Split into simpler sentences: Joseph MacManus' father, Seán MacManus, who at the time was a leading republican later became Mayor of Sligo. This was the first Sinn Fein Mayor in the Republic of Ireland since the beginning of the The Troubles in 1969.