Short answer: Yes.
Longer answer: I've had an Asterisk system running on EC2 for the last 18 months, beginning shortly after this question was asked; it has never had a significant call quality issue caused by being on EC2. It provides listener call-in lines and business lines for a nationally syndicated US talk radio show, with four lines into the studio and an indefinite number in queue beyond that; and if there is a call quality issue I have the host breathing down my neck in very short order.
The caveats given by @voretaq7 in his answer apply, of course.
You must have a reliable timing source for things like conference calls and music-on-hold to work properly. (The talk radio show uses MOH.) Fortunately the dahdi driver is able to get reliable enough timing from the virtualized USB subsystem, which is their timing source fallback when a line card is not present in the system.
You also must minimize latency, as much as possible. With the Asterisk server in the US-East Amazon region, I'm getting about 28-30ms latency to the ATAs in the studio, as reported by
sip show peers, which is about the best I can do because of where the studio is physically located. Anything above that, as previously noted, is likely to cause quality issues.
In your case, the latency to Ireland and back is likely to kill this idea, though you didn't give any specific measurement of the latency so it's hard to be sure. If you use at least a small instance you aren't likely to have any CPU issues even with 20 channels.