The pollution is coming from nearby farms.
Westmeath features on a list of the worst quality bathing waters in Ireland.
The Environmental Protection Agency's annual report shows 111 out of 148 were deemed "excellent" in 2020 - an increase of four on 2019.
While four locations were classed as having 'poor' water quality - including Lilliput at Lough Ennell.
It's now had a bathing restrictions for three years.
Mary Gurrie of the EPA outlines the worst affected areas in Ireland:
The EPA says Lough Lene and Lough Owel were both found to be of good quality.
But authorities in Westmeath are still trying to identify the source of contamination at Lough Ennell.
The area is facing declassification if the issue isn't resolved in another two years.
The EPA's Marie Archbold says the bathing restrictions are in place as a result of pollution from nearby farms.
She explains why it's such a stubborn problem to solve:
People are advised Lilliput is not safe for swimming, especially after a lot of rainfall when bacterial contamination may be higher.
Prison Dispute Linked To Fatal Arson Attack In Offaly
BnM Opens New Funding Round For Offaly Community Benefit Fund
Orange Wind Alerts Issued As Storm Bram Approaches
Irish Hauliers Threaten Protests Over Toll Discount Dispute
Former Irish Soccer Legend Backs Offaly Community After Arson Attack
Tributes Paid To Former Fine Gael Midlands Politician
Midlands 103 Launches Joint Christmas Appeal With Simon Community
Offaly Arson Attack Is An Attack On Society Itself - Taoiseach
Woman Facing Life In Prison After Double Midlands Murder
Housing Committee Chair Flags Cost Concerns Over Apartment Fire Safety Scheme
Justice Minister Wants People To Come Forward Over Offaly Arson Attack
New Report Warns Of “Poor Liquidity” As Ireland’s Housing Market Stalls
Laois TD Renews Calls For Updated Wind Farm Guidelines
Midlands Man Laid To Rest Today
Anti-Racism Funding Granted For Offaly And Westmeath Groups
Gardaí Begin Murder Investigation After Child And Woman Dies In Offaly House Fire
Ashling Murphy Murder "Wrongly Accused" Granted Bail Over Theft Charge
Young Boy And Woman Killed In Offaly House Fire
Laois Makeup Artist Warns Of Expired Suncream Dangers
Lynx and Wolves In Ireland Pose Minimal Threat - Offaly Biodiversity