Complex sentence: The Uí Fhearghail went on to become princes of the territory of "Anghaile" (Annaly), roughly corresponding to present day County Longford, their chieftain sitting at "Longphort Uí Fhearghail" (O'Farrell's stronghold), present day town of Longford.
Split into simpler sentences: The Uí Fhearghail went on to become princes of the territory of "Anghaile" (Annaly), roughly corresponding to present-day County Longford. Their chieftain sat at "Longphort Uí Fhearghail" (O'Farrell's stronghold), the present-day town of Longford.