Welcome hepperterry,
the term 'expansion' (especially when it comes along as 'city expansion') is mostly associated with cultural border expansions of a city.
If you refer to such expansion definition, mining or irrigation does not help, but any cultur point producing city improvement ('building') does, e.g. temple, library.
Now mining would generally help to complete such a building faster, plus irrigation in the early game suffers from the despotism tile penalty (irrigating a grassland tile w/o bonus food is totally useless, for example), so you could say that mining would lead to an earlier border expansion. OTOH, growth is very important, so you would want to irrigate when you could harvest more food in the process for faster growth (bigger city -> more working tiles -> more production).