If not specified in the URL, the default driver for the mssql+pyodbc dialect would be “SQL Server” [1]. That means you need to have a section that reads like this in /etc/unixODBC/odbcinst.ini:
[SQL Server]
Driver=/path/to/library.so
It works “automatically” on Windows, because if you open Administrator Tools -> Data Sources (ODBC), you would most likely find an entry named “SQL Server” under the Drivers tab.
On Linux, you can either use the FreeTDS driver, or the official driver from Microsoft (I recommend this).
After installing the driver, you should have something like this in /etc/unixODBC/odbcinst.ini:
[FreeTDS]
Driver=/usr/lib/libtdsodbc.so
Threading=1
[ODBC Driver 11 for SQL Server]
Description=Microsoft ODBC Driver 11 for SQL Server
Driver=/opt/microsoft/msodbcsql/lib64/libmsodbcsql-11.0.so.2270.0
Threading=1
UsageCount=1
Then, you just have to add a driver query string parameter to the URL, with value that matches the section name.
Sample URL with FreeTDS:
mssql+pyodbc://user:pass@ip_address/database_name?driver=FreeTDS
Sample URL with the official driver:
mssql+pyodbc://user:pass@ip_address/database_name?driver=ODBC+Driver+11+for+SQL+Server
[1] https://bitbucket.org/sqlalchemy/sqlalchemy/src/aa3a8f016f3e4396d125b18b0510abdf72aa8af2/lib/sqlalchemy/dialects/mssql/pyodbc.py?at=default#cl-236