functions.php

<?php

/**
 * Insert after Paragraph
 * When running on the_content, use priority > 20 so it doesn't affect oEmbed
 * 
 * @param string $insertion
 * @param int $paragraph_id
 * @param string $content
 * @return string $modified_content
 */
function ea_insert_after_paragraph( $insertion, $paragraph_id, $content, $include_at_end = true ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, wpautop( $content ) );
	foreach ($paragraphs as $index => $paragraph) {

		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}

		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
			$inserted = true;
		}
	}

	if( ! $inserted && $include_at_end )
		$paragraphs[] = $insertion;
	
	return implode( '', $paragraphs );
}

Hey Bill. Where does $inserted come from, on line 26?

@JiveDig sorry about that, just now saw this.

I added $inserted on line 23. The idea is if the paragraph ID you specify is greater than the number of paragraphs total, then add the insertion at the end of the content.