Guitar tuning
Most guitars are tuned in the standard fashion EADGBE (from thickest string to thinnest). This is fine and many tunes can be played, but experimenting with open tunings reveals new depths, new harmonies and new ways of playing. Open tunings allow arrangements that would be difficult or perhaps impossible in standard tuning. Here are a few of the tunings I use.

You can get a spreadsheet allowing easy access to all Carolan tunes in any particular tuning here.

The Open C Tunings	The scary part here is tuning the 2nd string (normally pitched at B) UP to C. It's only a semi-tone higher and is quite safe.
Open C - CGCGCE	Tunes in this tuning include 018 Sir Charles Coote and 163 Captain Sudley / Caroan's Dowry (written for the marriage of his daughter)
Open Cm - CGCGCEb	Tunes in this sad sounding tuning include 024 Mrs Crofton and 087 The Princess Royal
Open C9 - CGCGCD	Tunes in this plaintive sounding tuning include 037 Fanny Dillon and 191 Carolan's Ramble to Cashel
Double Cm - CGEbGCEb	You get a Cm chord from the top three and the bottom three strings - see 172 Carolan's Lament
The Open G Tunings	These are very like the tunings of normal banjo
Open G - DGDGBD	Tunes in this tuning include 156 Mervyn Pratt and 213 Squire Wood's Lament (on the Refusal of his Halfpence)
Open Gc - CGDGBD	Tunes in this tuning include 046 Mrs Fallon and 072 John Kelly
Open Gf - FGDGBD	Tunes in this tuning include 103 John Moore
Open GmD - DGDGBbD	Tunes in this tuning include 004 James Betagh and the very odd 201 Separation of Soul and Body
Open GmF - FGDGBbD	Tunes in this tuning include 003 Mrs Bermingham and 068 Mrs Judge
Open GmC - CGDGBbD	Tunes in this tuning include 135 Mrs O'Neill of Carlane and 145 Planxty O'Rourke, 2nd Air
Open GmDc - DGDGBbC	Tunes in this tuning include 093 Captain Magan
The Other Tunings
Dropped D - DADGBE	Tunes in this tuning include 150 Eleanor Plunkett
Cgac - CGCGAC	Tunes in this tuning include 039 Luke Dillon and 100 Lord Massarene
Dgac - DGDGAC	Tunes in this tuning include 122 Denis O'Conor 1st Air
Dgbbc - DGDGBbC	Tunes in this tuning include 133 Captain O'Kane
DADGBD	Tunes in this tuning include 073 Mabel Kelly and 202 Sheebag and Sheemor
DADF#AD	Tunes in this tuning include 123 Dennis O'Connor, 2nd Air and 161 Dr John Stafford / Carolan's Receipt (for Drinking)