Since this is SQL specific you will need to use an SQL Restriction within your criteria. For example:
def myinput = params.myinputfieldname
def results = Description.createCriteria().list() {
sqlRestriction "unaccent(content) ILIKE unaccent('%$myinput%')"
}
At least that's the idea.
Edit
Take care to note the fact 'content' in the restriction above refers to the actual SQL column name instead of the domain's field name.