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.
Midlands Hospice Planning Application Passed By Offaly County Council
Tolü Makay Joins Cast of DWTS Ireland
HSE Warn Flu Season Is Most Severe In Years
Laois Motorist Caught Speeding At 149 km/h
Offaly Woman Opposing Removal Of Roadside Memorial Removals
White Christmas Possible For Midlands
Midlands Campaign Urges Church To Explain McGregor Wedding Decision
Gardaí Search Suspected Westmeath Brothel
Offaly TD Backs Taoiseach Ahead Of Fianna Fail Campaign Review
Westmeath Distillery Showcased At European Parliament
Mental Health Charity Launches Festive Support Groups Over Christmas
Midlands Man Acquitted Of Father’s Murder On Grounds Of Insanity
I Hope My Son Has A Farm To Come Home To - IFA President
Ticket Prices Set To Rise For National Ploughing Championships
Taoiseach Warns Of Threats To Energy And Communication Cables
Irish Farmers To Join EU-Wide Protest In Brussels
Four Men Released Without Charge After Midlands Prison Assault Arrests
Community Mourns Mary Holt As Colleagues Lead Funeral Procession In Edenderry
Calls Grow For 24 Hour Garda Station In Edenderry After Arson Attack
Appeal Date Set For Jozef Puska Over Murder Of Ashling Murphy