The INMO says overcrowding and COVID-19 make for a "toxic combination".
Mullingar Regional Hospital is warning it's extremely busy today.
25 people are waiting on trolleys this afternoon - the third highest figure in the country.
Visitors are advised to only attend the emergency department if absolutely necessary, and not bring children if at all possible.
You're asked, where possible, to telephone your GP or pharmacist, rather than presenting at the hospital.
However, anyone seriously injured or ill will be assessed and treated as a priority.
Irish hospitals are seeing the highest number of people on trolleys since COVID restrictions were introduced in March.
221 people around the country are waiting for beds, according to the Irish Nurses and Midwives Organisation.
The union says overcrowding and COVID-19 make for a "toxic combination".