Rachel Kearns opened the scoring for Celtic two minutes into the second half with Jess Nolan doubling the host’s advantage on 65 minutes.
Orla Devanney secured their first win of the season with a clinical strike on 78 minutes, which moves Celtic to sixth in the table.
Meanwhile, champions Wexford Youths WFC continued their 100 per cent start to the season, but didn’t have it all their own way at home to Cork City WFC.
Cork goalkeeper Trish Fennelly denied Rianna Jarrett twice, Lorna O’Connell and Kylie Murphy as the first half ended scoreless.
Wexford introduced Claire O’Riordan and Carol Breen at half-time with Ashling Frawley opening the scoring for the hosts on the hour mark with a powerful run through the defence.
O’Riordan hammered home her fourth League goal of the season on 75 minutes with Breen adding a third after seven minutes from time.
The Dublin derby between Peamount United and Shelbourne Ladies and the UCD Waves v Galway WFC game were both postponed due to unplayable pitches. Both games will be re-arranged in due course.
Wexford Youth’s Women 3-0 Cork City Women
Castlebar Celtic 3-0 Kilkenny United
UCD Waves P-P Galway WFC
Peamount United P-P Shelbourne