Liason of "il est allé"

Stephen, 

it is an example of an optional liaison. In everyday (informal) speech optional liaisons are often not pronounced. 

The following is from the first link below: 

Optional LiaisonsLiaisons facultatives 

 

 

….2. Between two-part verbal structures

**ils ont eu[eel zo(n) tu][eel zo(n) u]

je suis allé[zheu swee zah lay][zheu swee ah lay]

tu vas aller[tu vah zah lay][tu vah ah lay]

 

…

See the first link for the sound clips, and more on optional liaisons specifically. The other links provide further discussion of liaisons, including those that are always required and those that are ‘ forbidden ‘.

https://www.lawlessfrench.com/pronunciation/optional-liaisons/

 https://www.lawlessfrench.com/pronunciation/liaisons/

 https://www.lawlessfrench.com/pronunciation/required-liaisons/

 https://www.lawlessfrench.com/pronunciation/forbidden-liaisons/