Unlike mutual funds, an ETF must be bought in whole shares.  (Mutual
funds can be bought in partial shares, and therefore you can reinvest
all dividends, even if your balance does not add up to a total share.)

Depending on your broker or account, dividends from a stock or ETF
should be paid into a cash balance within the IRA.  If these dividend
gains remain within the IRA account--i.e. you leave it as a cash
balance, you reinvest it, or you use it to pay for commission costs--
then you will not pay tax, provided you will not currently pay taxes
because the gains are still within the IRA balance.  (For traditional
IRA, you will pay taxes when your funds are finally disbursed from the
account.  For a Roth IRA, you will not pay taxes on gains if you
disburse your account within certain conditions.)