I am a Senegalese citizen, living in France. I was in Ireland two months ago, and my Irish visa has not yet expired. I am in France now, and I want to go to London. Can I travel there from France using the same Irish visa?
No. There is a joint British-Irish visa scheme that allows Chinese and Indian citizens to use an Irish visa to enter the UK or vice versa; however, this scheme does not apply to you as a Senegalese citizen.
Under the British-Irish visa scheme, some Irish short stay visas will allow onward travel to the UK and some UK visitor visas will allow onward travel to Ireland. For example, under the scheme an Indian or Chinese visitor in Dublin will be able to make a short trip to London or Belfast without needing a separate visa. Alternatively an Indian or Chinese visitor in London could travel to Dublin or Cork.
Note that even if this scheme did apply to you, it only allows "onward travel". In other words, if you hold an Irish visa and were in France, you could not travel directly to London using an Irish visa, but would need to enter the UK from Ireland instead.
Yes you will need a visa as a Senegalese citizen. Your Irish visa will not permit entry to UK.