The parish of Killasser is situated about one mile north of Swinford. The parish takes its name Killasser from Cill Lasrach - the church of Lasair. Lasair was an eight-century Irish saint who built a church in Knockmullin townland. The parish is rich in archaeological field monuments with Court Cairns in Cartronmacmanus, Coolagagh, Cregaun, a crannog in Lower Lough Callow, nearly 200 ring forts and several Fulachta Fiadh (Bronze Age cooking sites.).

The oldest church ruins in the parish are in the townland of Killshesnaun. It was built by Sheshnan O Ruane, who was a descendant of the local chieftains the O Ruanes. According to local folklore Sheshnan was a handsome young man who enjoyed a good time in his younger life. As he grew older he changed his ways and went to Rome to beg forgiveness from the Pope.

The Pope was impressed by this fine young man and invited him to take a seat and sit down. Sheshnan declined the offer until such time as he had confessed his sins. The Pope requested him to build a church in his own parish as an act of penance for his sins. This he did and he acted as its caretaker for the rest of his life.

The parish suffered a decline in population as a result of the famine and emigration over the years. In 1838 the total population was almost 7,000, while the present day population is only approximately 900.

The area is renowned for its fishing waters in particular the Callow lakes which yield excellent brown trout and the world renowned River Moy for salmon. The Killasser area also boasts two fine visitor centres Carraig Abhainn Open Farm and Hennigans Heritage Centre.