Pass stdin into Unix host or dig command

Use xargs -l:

tail -f access.log | xargs -l host

You could also use the read builtin:

tail -f access.log | while read line; do host $line; done

In the commands below, replace cat with tail -f, etc. if needed.

Using host:

$ cat my_ips | xargs -i host {}
1.1.1.1.in-addr.arpa domain name pointer myhost1.mydomain.com.
1.1.1.2.in-addr.arpa domain name pointer myhost2.mydomain.com.

Using dig:

$ cat my_ips | xargs -i dig -x {} +short
myhost1.mydomain.com.
myhost2.mydomain.com.

Note that the -i option to xargs implies the -L 1 option.

To first get the IPs of one's host, see this answer.