I would put the date that you can prove you entered the UK.
If you still have your boarding pass for your flight to Dublin, then you can put that date, and include the boarding pass as evidence that you entered the UK on that date.
If you don't have the boarding pass, it might be easier to put the Brussels date and show the stamp as evidence.
Or you could put the Dublin date and then explain that you got an entry stamp when you went to Brussels and came back into the UK.
Honestly, though, I don't think it really matters which date you put because the date is only an issue if you are trying to prove that you meet a residency requirement for your next visa - for example, if you had to prove you had been living in the UK for exactly 5 years for ILR... but you had no stamp to prove what date your 5 years started.
As you are just switching from one visa to another, and your 5-year ILR residency period won't start until you have your FLR(M), it won't really be an issue which date it was.
