You can play realistic celtic harp chords, thirds, fifths and octaves with this app.

Each square on the screen play a note. As you move across the screen,
notes are spaced one note apart, according to the selected key.
As you move up and down the screen, the notes go up and down an octave
per row.

Each corner of the square will play a different type of note cluster.
The lower left corner will play a rolled chord. The lower right corner will play
a third. Upper left - fifths, and the upper right, octaves.

Note that in the full (paid) version, you can change
keys. Changing the key will change which notes show up, whether those notes
are tuned to flat, natural or sharp, and whether the third interval or chord
is major or minor for a given note.

The paid version also has a few more notes you can play.

For best sound quality on the iPhone, headphones are recommended.

Thank you for looking, and hope you have a great day!