The main sense of dare is "not afraid" to do something.
Macmillan Dictionary provides definitions for both I dare say and dare I say (it).
I dare say British spoken
used for saying that something is probably true, although you do not know for certain
dare I say (it) mainly British spoken formal
used when you are saying something that you think other people may not like
This famous novel is a little, dare I say it, dull.
I dare say that is enough to understand the usages of the two.
Typically, I personally don't say either I dare say (sometimes written I daresay) or dare I say (it).
Instead of "I dare say", I usually say these:
I would say ... (or I'd say) - I use this to express my opinion, without much assertive
I say ... - This is a bit stronger version of I would say or I dare say, but still leave room for uncertainty
If I think I'm sure, chances are I won't say I'd say, I dare say, or I say, because it's unnecessary.
Swan also suggested this in his Practical English Usage,
And I dare say (sometimes written I daresay) is used to mean 'I think probably', 'I suppose'.
Instead of "dare I say (it)", I usually say these (note that their meanings are not quite identical, but in appropriate contexts, they're quite interchangeable):
let me say this,
let me be blunt,
to put it bluntly,
let's face it,
Let me be blunt. This famous novel is a little dull.
To put it bluntly, this famous novel is a little dull.
This famous novel is a little, let's face it, dull.