We have several harpists join us at our jam at O'Hurleys in Shepherdstown, WV and they play on almost every tune (Irish and Old Time). Both harpists have full sharping levers, so the key is not a problem, but even then they both will sometimes provide a drone by playing the bass strings with a fanning movement not unlike a sting bass pluck and slap movement, or running a kind of bass line on fast pieces like "Groweling and Grumbling", or "Morrisons".

Also its not hard to convince the other musicians to play in your key(s) if you think ahead and provide some bridge chords (or notes) to run from the common key to yours. Then as the piece ends in one key, you bridge the transition into "your" key and let some of the softer instruments (like flutes, and the lutes, or classical guitars) follow you into the piece.

I can't tell where you are from, but I will suggest some local favorites to bridge.

"Captain Henry O'Kain" into or out of "Rights of Man".
"Arran Boat Song" into "Manx Waltzes"
"Fanny Power" into "Young Jane" into "Gentle Maiden"
And if you are into jigs, just run "Morrisons Jig" into "Swallowtail Jig" into "Road to Lisdonvarna", all in Em.

Finally, try to work out some pieces with a fiddler or flutist which allow the harp to shine, Like "Archibald MacDonald of Kepoch", "Bonnie at Morn", "Star of the County Down", "Gallacian Waltz" or "The Shearin's No For You".

An excellent series of Irish music books for harpists are the Sylvia Woods series "Irish Dance Tunes for All Harpists" and "40 O'Carolan Tunes for All Harps" by Sylvia Woods, Woods Music and Books, P.O. Box 816, Montrose, California, 91021, USA. We were introduced to them by the harpists, and they are now considered source books by all the musicans at the Thursday night jam.