From Athens:

Internal flight from Athens airport to Skiathos and then connection with a ferry or flying dolphin for your onward journey to Alonissos.
Flying dolphin or ferry from the port of Agios Konstantinos (165 km from Athens). The travel time is about 3,5 hours by flying dolphin and 5 hours by ferry.
Ferry from Kymi (160 km from Athens). The travel time is around 2,5 hours.
Ferry from Mantoudi (135 km from Athens). The travel time is 2 hours and 20 minutes.

From Skiathos:

There are numerous journeys each day. The travel time is around 3 hours by ferry and just over 1 hour by flying dolphin.

From Volos:

You will find several departure times each day. The travel time by ferry is approximately 5-6 hours and 2 hours 40 minutes by flying dolphin.

From Thessaloniki:

Direct service from Thessaloniki with Hellenic Seaways – the travel time is just under 4 hours.