United enjoyed a good 15-minute spell during which central midfielder Kevin Gannon displayed his authority.
They went close to reducing the margin in the 52nd minute when Gannon picked out substitute Adam Mulchrone with a brilliant penetrating pass, but Adam Chambers was swift off his line to deny him.
The introduction of Sam Hastings helped Celtic to consistently exploit space down the left flank throughout the second half.
However, the Castlebar attack disappointingly failed to take advantage of the probing crosses he played into the United penalty area.
A corner kick by Hastings led to Celtic's third goal before the end. He picked out Matthew Ruane who headed the ball with precision past McHale from four yards.
While it was far from a vintage performance by Celtic, it extended the side's winning run to 23 games, a run that started last April.
The minds of the players were clearly on the forthcoming Schoolboys FAI Under 14 Cup quarterfinal clash against Cherry Orchard.
Celtic's most impressive players on this occasion were Sean Conlon, Matthew Ruane, Philip Ruddy and Sam Hastings.
Best for Westport were McHale, Dan Lavelle, Niall Feehan, Gannon and Mulchrone.
Celtic will face Salthill Devon, Manulla or Ballaghaderreen in the final on the May bank holiday weekend.
Castlebar Celtic 3
Westport United 0
Castlebar Celtic: A. Chambers, R. Collins, M. Howley, S. Conlon, M. Ruane, S. McDermott, P. Ruddy, G. Walsh, N. Baba, M. Sweeney, J. Hunt.
Subs used: C. Lisibach, S. Hastings, D. Ralph and D. Murray.
Westport United: T. McHale, J. Ring, M. McDonagh, D. Lavelle, N. Feehan, B. Spring, K. Gannon, P. Moran, D. McGreal, J. Brennan, R. Nugent.
Sub used: A. Mulchrone.
Ref: M. Halligan (Castlebar).