No. The two are unrelated.
PHI/Income continuance/Salary protection is insurance against loss of income from your job/trade.
S.I. cover is protection against a "dread" event, mainly cancers and heart problems.
If you have good PHI and adequate health insurance (VHI/BUPA/VIVAS) additional SI cover (which won't qualify for tax relief*) might not be required.
*I've seen some SI cover in Occ. pension schemes although it's Revenue limited.