This article describes another simple Celtic harp I just completed.
It has 22 wire strings and a nice sized wooden soundbox.
I intend to write more about it's construction here but will post
what I have so far. This thing cost me very little. Main expenses
were: small part of a sheet of 5mm finished-one-side plywood from 
Home Depot; a few dollars worth of "Zither" tuning pins from 
elderly.com ;a couple of pieces of hardwood obtained from Habitat
for Humanity for a buck apiece; various gauge metal guitar strings
(friends give me their old strings) and a few other inexpensive odds
and ends (brads, screws glue etc.)

Additional notes:

The photos and illustrations should show well how to build this harp.
Without frets, curves etc, not much is critical! Maintain uniform
string-spacing, gauge & length (as per my dimensions) and all should 
be ok.

I do so much experimenting that I find it useful to buy rolls of 
"piano wire" aka "music wire" in various gauges. This is not that
expensive - ie: a fat roll of .014 wire costs just over ten dollars.
elderly.com carries some. 

Rock (aka hard) maple must be used for the pin-block piece. It's the
only common hardwood that will hold zither tuning pins without 
eventually slipping! Even hard oak slips! Cabinet shope can get you 
a piece for a few bucks. Be sure to use the recommended size pilot hole
for these tuners.

More info to follow.
Let me know if you build one of these things!
They sound and play every bit as good as the commercially made
wire-strung Celtic harps I've seen & are straightforward/easy
to build -- no curves to deal with etc.

The construction diagram here looks quite "hectic" -- just my attempt
to get it all on one page. One step at a time construction makes this
project flow easily tho.

String gauges:
I have been experimenting with lots of various string-gauge configurations,
having settled on these (from thickest to thinnest, in thousandths of an 
inch) - .038 wound; .026 wound; .026 wound; .025 wound; .024 wound; 
.020; .020; .020; .020; .020; all of the remaining 12 highest-pitched 
strings are .013. Obviously, feel free to experiment. A gauge or so 
either way should work. 

The lowest pitched string is tuned to the C note, one octave below 
middle C on a piano. The highest pitched string is tuned to the C note,
two octaves above piano's middle C. So tuning is, low to high:
c d e f g a b c d e f g a b c d e f g a b c

Even steel strings need a "stretching-in" period of time before everything
stays in tune. At first, expect to have to retune the beast every time you 
play. After a week or so everything stays pretty much in tune.

Dennis Havlena - 8/9/2011  
Webpage at www.DennisHavlena.com

Click here to access
my webpage