My metronome has a tap in function which is very useful for getting tempos - you can also see how steady the tempo is.

Hornpipe tempos seem to vary quite widely. Certainly here in NE England they are played quite quickly. The tempo of the Liverpool Hornpipe on the clip Jack posted just above is just under minim=80 and if anything that is a little slower than they tend to be played here. I did a quick check on my VotP CDs. On Rantin' and Reelin' which is mainly Northumbrian musicians, the hornpipes varied from minim=90 (Will Atkinson, and rock steady) to around minim=108 which is pretty quick. quicker than I would like to attempt. on Rig a Jig Jig which is music from Southern England there were only three hornpipes the tempos were minim=80 for two and minim=70 for the third.

The Connemara example posted by Dick above the fiddler was playing a rock steady minim=59

As to the rhythm, I know hornpipes are frequently notated in 4/4 but when I hear them played, they feel like two in a bar to me, and I noticed that the fiddler in the Connemara clip was tapping two in a bar even at that slow tempo. On the other hand, since hornpipes are normally played dotted, writing them out in 12/8 does give a reasonable indication of how they are played.

As an interesting aside to that last comment, someone in a session I go to in Durham recently brought in a tune called "Fools Jig" which he had got out of a tune book and which was claimed on the page was a morris dance tune - it may well have been, but when he played it I had this feeling of familiarity. The tune was in fact Roxburgh Castle transposed into D and written out as a jig!